Well it seems that I cant rely on this at all since every JPA provider does this differently.

I tried it with Batoo-JPA as well now and it didn't work either (even though Batoo doesn't seem as a good choice of a JPA provider in general, since it's not being updated that fast and I even got a ClassCastException in its Criteria API implementation).

Well, that's not that big of a big deal, to be honest. I will just stick with having to implement custom event providers for every JPA I'd like to support.
It could have been awesome if the event system would be mature, but apparently it isn't.