I asked a similar sort of question in this topic:
http://forums.hibernate.org/viewtopic.php?t=986784
The book basically says you can't use the EM during a callback, by definition of the standard.
The same restriction exists for the traditional Hibernate Session, but the workaround is pretty easy.
As, you can see from the topic though - I've no idea how to create a new EntityManager on the same connection. At the moment, my workaround is just to drop back to using a brand new Hibernate Session. It's a bit dodgy though, swapping from JPA API and back to the Hibernate API just because I can't figure out how to create an EM.