I've tried that already. My Stateless Session Bean has only one field which is an injtected instance of EntityManager. I added a @PReDestroy method to set it to null, but debugging while undeploying, I realized that it was null already.
One thing I came across is that if your hibernate entities contains enums (@Enumerated) then all of them get stuck in memory. If you remove all the enums from there, then they get collected after undeployment. Unfortunatelly, SLSB, proxies and $__javassist__$, are never removed. They stay in the current ClassLoader with no instance associated.
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