http://en.wikipedia.org/wiki/Haiku_(operating_system) seems to have a fair amount of docs on how their API works. Since it is an open source implementation of BeOS it might have some good information. BeOS was made with SMP in mind from word one.
I'm having a hard time guessing the details of how a GUI that doesn't have an event loop in it would work, so I think a more detailed answer would be very interesting.