Actually, the program you demonstrate is nonequivalent to the one I posted. The one I posted pickles just fine because 'bar' is a global name, but doesn't unpickle because it doesn't exist in the parent's namespace. (See http://docs.python.org/library/pickle.html#what-can-be-pickled-and-unpickled.) Although, if you're able to run my test program verbatim, then it's entirely possible I'm just missing something.
Anyway, I do think that adding a 'worker_missing_callback' could work. You'd still have to make sure the ApplyResult (or MapResult) can crash the pool if it deems necessary though.