I can't get triggering on draft-published to work.
This seems to be a problem in the implementation of the event firing in Gerrit itself. The event is not visible to other users. I will upload a patch on Gerrit.
Not a defect in the trigger plugin. Needs to be fixed in Gerrit.
Separate issue report is raised on Gerrit:
Please try it in Gerrit 2.5-rc2