I've looked through herald rules, and I can't seem to figure out how I can trigger a harbormaster build specifically when a differential request has been 'accepted'.
Is there a particular reason you need this? Generally, you would just have a Herald rule that would trigger every time a revision is submitted/updated, and nothing more.
Are you worried about putting to much work on your buildbots, or something?
EDIT: to make a Herald rule that triggers on every diff update, do this: Go to Herald, create a new Herald Rule. Create a rule for "Differential Revisions" that is "Global". On the conditions section, set the rule to:
"When all of these conditions are met: Repository, Is any of, <name here>"
The action is:
"Take these actions every time this rule matches: Run a build plan, <build plan name here>"
That's it, providing you have drydock configured correctly otherwise.