I suppose uae source can be compiled moreless everywhere ( not the one for windows, unfortunatly ). But it has different features depending from the platform ( for example, the one for linux is not as good as ( quite far from ) the one for windows ).
Anyway, in mess 0.62 it still doesn't work and it's called a TESTDRIVER ( to compile it, i changed TESTDRIVER to DRIVER, but the message was unable to start emulation ).
But modifying uae to satisfy mess-mame standards ( e.g. graphics output ) for me could have been a better option than rewriting it from scratch ( in the mess page, they write: our primary purpose is compatibility, not speed - i've read it somewhere... )
But it's better to wait than to write...