From Tom Jenkinson
Would it make sense to maintain the same restrictions as JMS?
In JMS you can initiate a new transaction from onMessage, analogous to a method decorated with @Observes. If the method returns without completing the transaction then an error is logged and the transaction rolled back.
Flowing a transaction from an event producer to an event consumer isn't a great idea (it doesn't work in JMS either). But allowing a consumer to control their own transaction does seem to make sense to me.
Admittedly this is a gut reaction, I read through the Jira and the doc you linked to though, and used my JEE experience to draw analogies, do let me know if I got the wrong end of the stick please!