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.