It should be easy enough to implement (a flag in the eviction region that causes a removal rather than an eviction when triggered) but I'm concerned that this would confuse the whole purpose of eviction, as you said.
Again, is this something generic enough to build in as a feature? It can easily enough be worked around in client code.
Eviction is really designed around memory management, and thus is asynchronous, and really has no firm guarantees that eviction will happen at a certain moment.
If I were to implement this without any facilities in JBossCache, I would probably create an external timer and my own method call interceptor.
I took a stab at it, and it's a lot more difficult than I thought ...