Hi Marco,
Thanks very much for posting all of this!
Could you maybe clarify why you want to use application-managed persistence with jBPM?
You probably know this, but for clarification:
RESOURCE_LOCAL means that you want the entityManager (e.g. the persistence layer) to be managed by the application. Application-managed persistence is most often used by servlets, EJB's, and locally running java applications because these are in essence small programs that don't need the extra overhead that container-managed persistence brings with it.
These programs usually open a persistence context/connection, get the stuff they need, commit the stuff they changed, and clean it all up: there is no usually no need for fine-grained control transactions.
In my opinion, jBPM does need more fine grained control over transactions. It needs this because when objects/data (and which objects/data) are persisted is a more complicated question. JTA gives you this control. Furthermore, I think there might be cases when you want to commit some objects while leaving other objects in the persistence context (logging objects vs. process context information).
In short, 1. jBPM needs to be sure that the persistence context is always available, which would be more work for jBPM with application-managed/ (container-managed persistence just works.. :-)) and 2. jBPM needs fine-grained control over transactions, which is easier with JTA.
Does that make any sense? If I'm missing something, please point it out!