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
@@ -2915,6 +2915,7 @@
     'PhabricatorStandardCustomField' => 'infrastructure/customfield/standard/PhabricatorStandardCustomField.php',
     'PhabricatorStandardCustomFieldBool' => 'infrastructure/customfield/standard/PhabricatorStandardCustomFieldBool.php',
     'PhabricatorStandardCustomFieldCredential' => 'infrastructure/customfield/standard/PhabricatorStandardCustomFieldCredential.php',
+    'PhabricatorStandardCustomFieldDatasource' => 'infrastructure/customfield/standard/PhabricatorStandardCustomFieldDatasource.php',
     'PhabricatorStandardCustomFieldDate' => 'infrastructure/customfield/standard/PhabricatorStandardCustomFieldDate.php',
     'PhabricatorStandardCustomFieldHeader' => 'infrastructure/customfield/standard/PhabricatorStandardCustomFieldHeader.php',
     'PhabricatorStandardCustomFieldInt' => 'infrastructure/customfield/standard/PhabricatorStandardCustomFieldInt.php',
@@ -2924,6 +2925,7 @@
     'PhabricatorStandardCustomFieldRemarkup' => 'infrastructure/customfield/standard/PhabricatorStandardCustomFieldRemarkup.php',
     'PhabricatorStandardCustomFieldSelect' => 'infrastructure/customfield/standard/PhabricatorStandardCustomFieldSelect.php',
     'PhabricatorStandardCustomFieldText' => 'infrastructure/customfield/standard/PhabricatorStandardCustomFieldText.php',
+    'PhabricatorStandardCustomFieldTokenizer' => 'infrastructure/customfield/standard/PhabricatorStandardCustomFieldTokenizer.php',
     'PhabricatorStandardCustomFieldUsers' => 'infrastructure/customfield/standard/PhabricatorStandardCustomFieldUsers.php',
     'PhabricatorStandardPageView' => 'view/page/PhabricatorStandardPageView.php',
     'PhabricatorStandardSelectCustomFieldDatasource' => 'infrastructure/customfield/datasource/PhabricatorStandardSelectCustomFieldDatasource.php',
@@ -7022,6 +7024,7 @@
     'PhabricatorStandardCustomField' => 'PhabricatorCustomField',
     'PhabricatorStandardCustomFieldBool' => 'PhabricatorStandardCustomField',
     'PhabricatorStandardCustomFieldCredential' => 'PhabricatorStandardCustomField',
+    'PhabricatorStandardCustomFieldDatasource' => 'PhabricatorStandardCustomFieldTokenizer',
     'PhabricatorStandardCustomFieldDate' => 'PhabricatorStandardCustomField',
     'PhabricatorStandardCustomFieldHeader' => 'PhabricatorStandardCustomField',
     'PhabricatorStandardCustomFieldInt' => 'PhabricatorStandardCustomField',
@@ -7030,7 +7033,8 @@
     'PhabricatorStandardCustomFieldRemarkup' => 'PhabricatorStandardCustomField',
     'PhabricatorStandardCustomFieldSelect' => 'PhabricatorStandardCustomField',
     'PhabricatorStandardCustomFieldText' => 'PhabricatorStandardCustomField',
-    'PhabricatorStandardCustomFieldUsers' => 'PhabricatorStandardCustomFieldPHIDs',
+    'PhabricatorStandardCustomFieldTokenizer' => 'PhabricatorStandardCustomFieldPHIDs',
+    'PhabricatorStandardCustomFieldUsers' => 'PhabricatorStandardCustomFieldTokenizer',
     'PhabricatorStandardPageView' => 'PhabricatorBarePageView',
     'PhabricatorStandardSelectCustomFieldDatasource' => 'PhabricatorTypeaheadDatasource',
     'PhabricatorStatusController' => 'PhabricatorController',
diff --git a/src/applications/almanac/typeahead/AlmanacServiceDatasource.php b/src/applications/almanac/typeahead/AlmanacServiceDatasource.php
--- a/src/applications/almanac/typeahead/AlmanacServiceDatasource.php
+++ b/src/applications/almanac/typeahead/AlmanacServiceDatasource.php
@@ -23,6 +23,16 @@
       ->withNamePrefix($raw_query)
       ->setOrder('name');
 
+    // TODO: When service classes are restricted, it might be nice to customize
+    // the title and placeholder text to show which service types can be
+    // selected, or show all services but mark the invalid ones disabled and
+    // prevent their selection.
+
+    $service_classes = $this->getParameter('serviceClasses');
+    if ($service_classes) {
+      $services->withServiceClasses($service_classes);
+    }
+
     $services = $this->executeQuery($services);
 
     if ($services) {
diff --git a/src/docs/user/configuration/custom_fields.diviner b/src/docs/user/configuration/custom_fields.diviner
--- a/src/docs/user/configuration/custom_fields.diviner
+++ b/src/docs/user/configuration/custom_fields.diviner
@@ -144,6 +144,9 @@
   - **Credentials**: Controls with type `credential` allow selection of a
     Passphrase credential which provides `credential.provides`, and creation
     of credentials of `credential.type`.
+  - **Datasource**: Controls with type `datasource` allow selection of tokens
+    from an arbitrary datasource, controlled with `datasource.class` and
+    `datasource.parameters`.
 
 = Advanced Custom Fields =
 
diff --git a/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldDatasource.php b/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldDatasource.php
new file mode 100644
--- /dev/null
+++ b/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldDatasource.php
@@ -0,0 +1,28 @@
+<?php
+
+final class PhabricatorStandardCustomFieldDatasource
+  extends PhabricatorStandardCustomFieldTokenizer {
+
+  public function getFieldType() {
+    return 'datasource';
+  }
+
+  public function getDatasource() {
+    $parameters = $this->getFieldConfigValue('datasource.parameters', array());
+
+    $class = $this->getFieldConfigValue('datasource.class');
+    $parent = 'PhabricatorTypeaheadDatasource';
+    if (!is_subclass_of($class, $parent)) {
+      throw new Exception(
+        pht(
+          'Configured datasource class "%s" must be a valid subclass of '.
+          '"%s".',
+          $class,
+          $parent));
+    }
+
+    return newv($class, array())
+      ->setParameters($parameters);
+  }
+
+}
diff --git a/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldUsers.php b/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldTokenizer.php
copy from src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldUsers.php
copy to src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldTokenizer.php
--- a/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldUsers.php
+++ b/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldTokenizer.php
@@ -1,11 +1,9 @@
 <?php
 
-final class PhabricatorStandardCustomFieldUsers
+abstract class PhabricatorStandardCustomFieldTokenizer
   extends PhabricatorStandardCustomFieldPHIDs {
 
-  public function getFieldType() {
-    return 'users';
-  }
+  abstract public function getDatasource();
 
   public function renderEditControl(array $handles) {
     $value = $this->getFieldValue();
@@ -14,7 +12,7 @@
       ->setUser($this->getViewer())
       ->setLabel($this->getFieldName())
       ->setName($this->getFieldKey())
-      ->setDatasource(new PhabricatorPeopleDatasource())
+      ->setDatasource($this->getDatasource())
       ->setCaption($this->getCaption())
       ->setValue(nonempty($value, array()));
 
@@ -34,7 +32,7 @@
     $control = id(new AphrontFormTokenizerControl())
       ->setLabel($this->getFieldName())
       ->setName($this->getFieldKey())
-      ->setDatasource(new PhabricatorPeopleDatasource())
+      ->setDatasource($this->getDatasource())
       ->setValue(nonempty($value, array()));
 
     $form->appendControl($control);
@@ -43,7 +41,7 @@
   public function getHeraldFieldValueType($condition) {
     return id(new HeraldTokenizerFieldValue())
       ->setKey('custom.'.$this->getFieldKey())
-      ->setDatasource(new PhabricatorPeopleDatasource());
+      ->setDatasource($this->getDatasource());
   }
 
 }
diff --git a/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldUsers.php b/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldUsers.php
--- a/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldUsers.php
+++ b/src/infrastructure/customfield/standard/PhabricatorStandardCustomFieldUsers.php
@@ -1,49 +1,14 @@
 <?php
 
 final class PhabricatorStandardCustomFieldUsers
-  extends PhabricatorStandardCustomFieldPHIDs {
+  extends PhabricatorStandardCustomFieldTokenizer {
 
   public function getFieldType() {
     return 'users';
   }
 
-  public function renderEditControl(array $handles) {
-    $value = $this->getFieldValue();
-
-    $control = id(new AphrontFormTokenizerControl())
-      ->setUser($this->getViewer())
-      ->setLabel($this->getFieldName())
-      ->setName($this->getFieldKey())
-      ->setDatasource(new PhabricatorPeopleDatasource())
-      ->setCaption($this->getCaption())
-      ->setValue(nonempty($value, array()));
-
-    $limit = $this->getFieldConfigValue('limit');
-    if ($limit) {
-      $control->setLimit($limit);
-    }
-
-    return $control;
-  }
-
-  public function appendToApplicationSearchForm(
-    PhabricatorApplicationSearchEngine $engine,
-    AphrontFormView $form,
-    $value) {
-
-    $control = id(new AphrontFormTokenizerControl())
-      ->setLabel($this->getFieldName())
-      ->setName($this->getFieldKey())
-      ->setDatasource(new PhabricatorPeopleDatasource())
-      ->setValue(nonempty($value, array()));
-
-    $form->appendControl($control);
-  }
-
-  public function getHeraldFieldValueType($condition) {
-    return id(new HeraldTokenizerFieldValue())
-      ->setKey('custom.'.$this->getFieldKey())
-      ->setDatasource(new PhabricatorPeopleDatasource());
+  public function getDatasource() {
+    return new PhabricatorPeopleDatasource();
   }
 
 }