Mac OS X: is it possible to imbue a non-main thread to become "The Main Thread" of a process?
OK, so according to your comment: you basically won't escape refactoring of your code. Most GUI systems use main thread and handle event loops for themselves. But if you say that GUI is optional, maybe it'd better to split your application into two -- worker and GUI. GUI could communicate with worker via numerous ways, depending on platforms/specific needs.