diff --git a/src/__phutil_library_map__.php b/src/__phutil_library_map__.php
--- a/src/__phutil_library_map__.php
+++ b/src/__phutil_library_map__.php
@@ -806,6 +806,7 @@
     'HeraldCondition' => 'applications/herald/storage/HeraldCondition.php',
     'HeraldConditionTranscript' => 'applications/herald/storage/transcript/HeraldConditionTranscript.php',
     'HeraldController' => 'applications/herald/controller/HeraldController.php',
+    'HeraldCustomAction' => 'applications/herald/extension/HeraldCustomAction.php',
     'HeraldDAO' => 'applications/herald/storage/HeraldDAO.php',
     'HeraldDifferentialRevisionAdapter' => 'applications/herald/adapter/HeraldDifferentialRevisionAdapter.php',
     'HeraldDisableController' => 'applications/herald/controller/HeraldDisableController.php',
diff --git a/src/applications/herald/adapter/HeraldAdapter.php b/src/applications/herald/adapter/HeraldAdapter.php
--- a/src/applications/herald/adapter/HeraldAdapter.php
+++ b/src/applications/herald/adapter/HeraldAdapter.php
@@ -101,6 +101,24 @@
   private $contentSource;
   private $isNewObject;
   private $customFields = false;
+  private $customActions = null;
+  private $queuedTransactions = array();
+
+  public function getCustomActions() {
+    if ($this->customActions === null) {
+      $this->customActions = id(new PhutilSymbolLoader())
+        ->setAncestorClass('HeraldCustomAction')
+        ->loadObjects();
+
+      foreach ($this->customActions as $key => $object) {
+        if (!$object->appliesToAdapter($this)) {
+          unset($this->customActions[$key]);
+        }
+      }
+    }
+
+    return $this->customActions;
+  }
 
   public function setContentSource(PhabricatorContentSource $content_source) {
     $this->contentSource = $content_source;
@@ -145,7 +163,24 @@
     }
   }
 
-  abstract public function applyHeraldEffects(array $effects);
+  public abstract function applyHeraldEffects(array $effects);
+
+  protected function handleCustomHeraldEffect(HeraldEffect $effect) {
+    foreach ($this->getCustomActions() as $custom_action) {
+      if ($effect->getAction() == $custom_action->getActionKey()) {
+        $result = $custom_action->applyEffect(
+          $this,
+          $this->getObject(),
+          $effect);
+
+        if ($result !== null) {
+          return $result;
+        }
+      }
+    }
+
+    return null;
+  }
 
   public function isAvailableToUser(PhabricatorUser $viewer) {
     $applications = id(new PhabricatorApplicationQuery())
@@ -157,6 +192,14 @@
     return !empty($applications);
   }
 
+  public function queueTransaction($transaction) {
+    $this->queuedTransactions[] = $transaction;
+  }
+
+  public function getQueuedTransactions() {
+    return $this->queuedTransactions;
+  }
+
 
   /**
    * NOTE: You generally should not override this; it exists to support legacy
@@ -645,13 +688,21 @@
 
 /* -(  Actions  )------------------------------------------------------------ */
 
-  abstract public function getActions($rule_type);
+  public function getActions($rule_type) {
+    $results = array();
+    foreach ($this->getCustomActions() as $custom_action) {
+      if ($custom_action->appliesToRuleType($rule_type)) {
+        $results[] = $custom_action->getActionKey();
+      }
+    }
+    return $results;
+  }
 
   public function getActionNameMap($rule_type) {
     switch ($rule_type) {
       case HeraldRuleTypeConfig::RULE_TYPE_GLOBAL:
       case HeraldRuleTypeConfig::RULE_TYPE_OBJECT:
-        return array(
+        $standard = array(
           self::ACTION_NOTHING      => pht('Do nothing'),
           self::ACTION_ADD_CC       => pht('Add emails to CC'),
           self::ACTION_REMOVE_CC    => pht('Remove emails from CC'),
@@ -666,8 +717,9 @@
           self::ACTION_REQUIRE_SIGNATURE => pht('Require legal signatures'),
           self::ACTION_BLOCK => pht('Block change with message'),
         );
+        break;
       case HeraldRuleTypeConfig::RULE_TYPE_PERSONAL:
-        return array(
+        $standard = array(
           self::ACTION_NOTHING      => pht('Do nothing'),
           self::ACTION_ADD_CC       => pht('Add me to CC'),
           self::ACTION_REMOVE_CC    => pht('Remove me from CC'),
@@ -680,9 +732,19 @@
           self::ACTION_ADD_BLOCKING_REVIEWERS =>
             pht('Add me as a blocking reviewer'),
         );
+        break;
       default:
         throw new Exception("Unknown rule type '{$rule_type}'!");
     }
+
+    foreach ($this->getCustomActions() as $custom_action) {
+      if ($custom_action->appliesToRuleType($rule_type)) {
+        $standard[$custom_action->getActionKey()] =
+          $custom_action->getActionName();
+      }
+    }
+
+    return $standard;
   }
 
   public function willSaveAction(
@@ -814,7 +876,7 @@
     }
   }
 
-  public static function getValueTypeForAction($action, $rule_type) {
+  public function getValueTypeForAction($action, $rule_type) {
     $is_personal = ($rule_type == HeraldRuleTypeConfig::RULE_TYPE_PERSONAL);
 
     if ($is_personal) {
@@ -832,8 +894,6 @@
           return self::VALUE_FLAG_COLOR;
         case self::ACTION_ADD_PROJECTS:
           return self::VALUE_PROJECT;
-        default:
-          throw new Exception("Unknown or invalid action '{$action}'.");
       }
     } else {
       switch ($action) {
@@ -859,10 +919,18 @@
           return self::VALUE_LEGAL_DOCUMENTS;
         case self::ACTION_BLOCK:
           return self::VALUE_TEXT;
-        default:
-          throw new Exception("Unknown or invalid action '{$action}'.");
       }
     }
+
+    foreach ($this->getCustomActions() as $custom_action) {
+      if ($custom_action->appliesToRuleType($rule_type)) {
+        if ($action === $custom_action->getActionKey()) {
+          return $custom_action->getActionType();
+        }
+      }
+    }
+
+    throw new Exception("Unknown or invalid action '".$action."'.");
   }
 
 
diff --git a/src/applications/herald/adapter/HeraldCommitAdapter.php b/src/applications/herald/adapter/HeraldCommitAdapter.php
--- a/src/applications/herald/adapter/HeraldCommitAdapter.php
+++ b/src/applications/herald/adapter/HeraldCommitAdapter.php
@@ -139,21 +139,25 @@
     switch ($rule_type) {
       case HeraldRuleTypeConfig::RULE_TYPE_GLOBAL:
       case HeraldRuleTypeConfig::RULE_TYPE_OBJECT:
-        return array(
-          self::ACTION_ADD_CC,
-          self::ACTION_EMAIL,
-          self::ACTION_AUDIT,
-          self::ACTION_APPLY_BUILD_PLANS,
-          self::ACTION_NOTHING
-        );
+        return array_merge(
+          array(
+            self::ACTION_ADD_CC,
+            self::ACTION_EMAIL,
+            self::ACTION_AUDIT,
+            self::ACTION_APPLY_BUILD_PLANS,
+            self::ACTION_NOTHING
+          ),
+          parent::getActions($rule_type));
       case HeraldRuleTypeConfig::RULE_TYPE_PERSONAL:
-        return array(
-          self::ACTION_ADD_CC,
-          self::ACTION_EMAIL,
-          self::ACTION_FLAG,
-          self::ACTION_AUDIT,
-          self::ACTION_NOTHING,
-        );
+        return array_merge(
+          array(
+            self::ACTION_ADD_CC,
+            self::ACTION_EMAIL,
+            self::ACTION_FLAG,
+            self::ACTION_AUDIT,
+            self::ACTION_NOTHING,
+          ),
+          parent::getActions($rule_type));
     }
   }
 
@@ -544,7 +548,13 @@
             $this->commit->getPHID());
           break;
         default:
-          throw new Exception("No rules to handle action '{$action}'.");
+          $custom_result = parent::handleCustomHeraldEffect($effect);
+          if ($custom_result === null) {
+            throw new Exception("No rules to handle action '{$action}'.");
+          }
+
+          $result[] = $custom_result;
+          break;
       }
     }
     return $result;
diff --git a/src/applications/herald/adapter/HeraldDifferentialRevisionAdapter.php b/src/applications/herald/adapter/HeraldDifferentialRevisionAdapter.php
--- a/src/applications/herald/adapter/HeraldDifferentialRevisionAdapter.php
+++ b/src/applications/herald/adapter/HeraldDifferentialRevisionAdapter.php
@@ -345,26 +345,30 @@
   public function getActions($rule_type) {
     switch ($rule_type) {
       case HeraldRuleTypeConfig::RULE_TYPE_GLOBAL:
-        return array(
-          self::ACTION_ADD_CC,
-          self::ACTION_REMOVE_CC,
-          self::ACTION_EMAIL,
-          self::ACTION_ADD_REVIEWERS,
-          self::ACTION_ADD_BLOCKING_REVIEWERS,
-          self::ACTION_APPLY_BUILD_PLANS,
-          self::ACTION_REQUIRE_SIGNATURE,
-          self::ACTION_NOTHING,
-        );
+        return array_merge(
+          array(
+            self::ACTION_ADD_CC,
+            self::ACTION_REMOVE_CC,
+            self::ACTION_EMAIL,
+            self::ACTION_ADD_REVIEWERS,
+            self::ACTION_ADD_BLOCKING_REVIEWERS,
+            self::ACTION_APPLY_BUILD_PLANS,
+            self::ACTION_REQUIRE_SIGNATURE,
+            self::ACTION_NOTHING,
+          ),
+          parent::getActions($rule_type));
       case HeraldRuleTypeConfig::RULE_TYPE_PERSONAL:
-        return array(
-          self::ACTION_ADD_CC,
-          self::ACTION_REMOVE_CC,
-          self::ACTION_EMAIL,
-          self::ACTION_FLAG,
-          self::ACTION_ADD_REVIEWERS,
-          self::ACTION_ADD_BLOCKING_REVIEWERS,
-          self::ACTION_NOTHING,
-        );
+        return array_merge(
+          array(
+            self::ACTION_ADD_CC,
+            self::ACTION_REMOVE_CC,
+            self::ACTION_EMAIL,
+            self::ACTION_FLAG,
+            self::ACTION_ADD_REVIEWERS,
+            self::ACTION_ADD_BLOCKING_REVIEWERS,
+            self::ACTION_NOTHING,
+          ),
+          parent::getActions($rule_type));
     }
   }
 
@@ -491,7 +495,13 @@
             pht('Required signatures.'));
           break;
         default:
-          throw new Exception("No rules to handle action '{$action}'.");
+          $custom_result = parent::handleCustomHeraldEffect($effect);
+          if ($custom_result === null) {
+            throw new Exception("No rules to handle action '{$action}'.");
+          }
+
+          $result[] = $custom_result;
+          break;
       }
     }
     return $result;
diff --git a/src/applications/herald/adapter/HeraldManiphestTaskAdapter.php b/src/applications/herald/adapter/HeraldManiphestTaskAdapter.php
--- a/src/applications/herald/adapter/HeraldManiphestTaskAdapter.php
+++ b/src/applications/herald/adapter/HeraldManiphestTaskAdapter.php
@@ -98,21 +98,25 @@
   public function getActions($rule_type) {
     switch ($rule_type) {
       case HeraldRuleTypeConfig::RULE_TYPE_GLOBAL:
-        return array(
-          self::ACTION_ADD_CC,
-          self::ACTION_EMAIL,
-          self::ACTION_ASSIGN_TASK,
-          self::ACTION_ADD_PROJECTS,
-          self::ACTION_NOTHING,
-        );
+        return array_merge(
+          array(
+            self::ACTION_ADD_CC,
+            self::ACTION_EMAIL,
+            self::ACTION_ASSIGN_TASK,
+            self::ACTION_ADD_PROJECTS,
+            self::ACTION_NOTHING,
+          ),
+          parent::getActions($rule_type));
       case HeraldRuleTypeConfig::RULE_TYPE_PERSONAL:
-        return array(
-          self::ACTION_ADD_CC,
-          self::ACTION_EMAIL,
-          self::ACTION_FLAG,
-          self::ACTION_ASSIGN_TASK,
-          self::ACTION_NOTHING,
-        );
+        return array_merge(
+          array(
+            self::ACTION_ADD_CC,
+            self::ACTION_EMAIL,
+            self::ACTION_FLAG,
+            self::ACTION_ASSIGN_TASK,
+            self::ACTION_NOTHING,
+          ),
+          parent::getActions($rule_type));
     }
   }
 
@@ -200,7 +204,13 @@
             pht('Added projects.'));
           break;
         default:
-          throw new Exception("No rules to handle action '{$action}'.");
+          $custom_result = parent::handleCustomHeraldEffect($effect);
+          if ($custom_result === null) {
+            throw new Exception("No rules to handle action '{$action}'.");
+          }
+
+          $result[] = $custom_result;
+          break;
       }
     }
     return $result;
diff --git a/src/applications/herald/adapter/HeraldPholioMockAdapter.php b/src/applications/herald/adapter/HeraldPholioMockAdapter.php
--- a/src/applications/herald/adapter/HeraldPholioMockAdapter.php
+++ b/src/applications/herald/adapter/HeraldPholioMockAdapter.php
@@ -64,16 +64,20 @@
   public function getActions($rule_type) {
     switch ($rule_type) {
       case HeraldRuleTypeConfig::RULE_TYPE_GLOBAL:
-        return array(
-          self::ACTION_ADD_CC,
-          self::ACTION_NOTHING,
-        );
+        return array_merge(
+          array(
+            self::ACTION_ADD_CC,
+            self::ACTION_NOTHING,
+          ),
+          parent::getActions($rule_type));
       case HeraldRuleTypeConfig::RULE_TYPE_PERSONAL:
-        return array(
-          self::ACTION_ADD_CC,
-          self::ACTION_FLAG,
-          self::ACTION_NOTHING,
-        );
+        return array_merge(
+          array(
+            self::ACTION_ADD_CC,
+            self::ACTION_FLAG,
+            self::ACTION_NOTHING,
+          ),
+          parent::getActions($rule_type));
     }
   }
 
@@ -133,7 +137,13 @@
             $this->getMock()->getPHID());
           break;
         default:
-          throw new Exception("No rules to handle action '{$action}'.");
+          $custom_result = parent::handleCustomHeraldEffect($effect);
+          if ($custom_result === null) {
+            throw new Exception("No rules to handle action '{$action}'.");
+          }
+
+          $result[] = $custom_result;
+          break;
       }
     }
     return $result;
diff --git a/src/applications/herald/controller/HeraldRuleController.php b/src/applications/herald/controller/HeraldRuleController.php
--- a/src/applications/herald/controller/HeraldRuleController.php
+++ b/src/applications/herald/controller/HeraldRuleController.php
@@ -397,11 +397,15 @@
             $current_value = $action->getTarget();
             break;
           default:
-            $target_map = array();
-            foreach ((array)$action->getTarget() as $fbid) {
-              $target_map[$fbid] = $handles[$fbid]->getName();
+            if (is_array($action->getTarget())) {
+              $target_map = array();
+              foreach ((array)$action->getTarget() as $fbid) {
+                $target_map[$fbid] = $handles[$fbid]->getName();
+              }
+              $current_value = $target_map;
+            } else {
+              $current_value = $action->getTarget();
             }
-            $current_value = $target_map;
             break;
         }
 
diff --git a/src/applications/herald/controller/HeraldTranscriptController.php b/src/applications/herald/controller/HeraldTranscriptController.php
--- a/src/applications/herald/controller/HeraldTranscriptController.php
+++ b/src/applications/herald/controller/HeraldTranscriptController.php
@@ -354,13 +354,15 @@
           $target = $target;
           break;
         default:
-          if ($target) {
+          if (is_array($target) && $target) {
             foreach ($target as $k => $phid) {
               if (isset($handles[$phid])) {
                 $target[$k] = $handles[$phid]->getName();
               }
             }
             $target = implode(', ', $target);
+          } else if (is_string($target)) {
+            $target = $target;
           } else {
             $target = '<empty>';
           }
diff --git a/src/applications/herald/extension/HeraldCustomAction.php b/src/applications/herald/extension/HeraldCustomAction.php
new file mode 100644
--- /dev/null
+++ b/src/applications/herald/extension/HeraldCustomAction.php
@@ -0,0 +1,20 @@
+<?php
+
+abstract class HeraldCustomAction {
+
+  public abstract function appliesToAdapter(HeraldAdapter $adapter);
+
+  public abstract function appliesToRuleType($rule_type);
+
+  public abstract function getActionKey();
+
+  public abstract function getActionName();
+
+  public abstract function getActionType();
+
+  public abstract function applyEffect(
+    HeraldAdapter $adapter,
+    $object,
+    HeraldEffect $effect);
+
+}
diff --git a/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php b/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
--- a/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
+++ b/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php
@@ -2197,7 +2197,9 @@
     $this->setHeraldAdapter($adapter);
     $this->setHeraldTranscript($xscript);
 
-    return $this->didApplyHeraldRules($object, $adapter, $xscript);
+    return array_merge(
+      $this->didApplyHeraldRules($object, $adapter, $xscript),
+      $adapter->getQueuedTransactions());
   }
 
   protected function didApplyHeraldRules(