You might be able to use Inline to do this by opening a separate hidden window and forking off a thread to handle its messages (in C or C++). Incoming messages could then signal your main program. Inline works with Event, so you can define your own events and still be compatible with the Tk message loop.

update: just started playing with Event again, and it appears that Event isn't compatible with Tk, just with PerlQt. Anyway, the suggestion about the separate thread is still potentially useful.