Thanks for the reproducer, I can now see the problem in SE environment.
However, when I try the same in EE (getting Weld proxy of EM and calling unwrap), it seems to work. So I was wondering if there can be any additional factor(s) in play (Hibernate settings? WFLY magic?). I am not really familiar with Hibernate and especially bootstrapping it in SE.
I'll try to dig up some more info.