Basically i've just moved some code around. (I've just added comments to the old stuff so you can see where it comes from)

All the mail functions from ApplicationTransactionEditor went into the ApplicationTransactionEmailWorker
The mail functions from ManiphestTransactionEditor into ManiphestTransactionEmailWorker.

The actual changes are mainly in the TransactionEmailWorker::doWork() (which now uses a loop per user to render templates).
and queueing the worker task in ApplicationTransactionEditor::sendMail()

This currently works for me and will correctly render tasks/repos that are accessible to only some users on the notification list accordingly.
(If a user doesen't have permission she'll see "restricted project" or only a task number).

Currently broken is still the feedpublish stuff in the ApplicationTransactionEditor.
If this generally idea sounds good i'd just have to move the mail stuff from the other classes extending the ApplicationTransactionEditor and using emails (about 15) into workers.

I finally have some time to work on this, sorry it took so long! I generally like this approach, but want to try to make the change more gradually. In particular:

• I'm looking at doing daemons and mail changes separately, so we move to the daemons first and then start splitting mail out.
• Although it might make sense to move mail code out of TransactionEditors eventually, this doesn't feel very important to me. The classes are sometimes pretty large (and the base class is enormous) but the code feels fairly cohesive and consistent to me. I think we can leave the code where it is for now, and not have to introduce new classes in each application.