I have been testing with (sdl)gnuboy and a custom gui about half a year ago. It worked fine (fully stretch screen, custom keys etc) and only had to do some more control settings fixes in the gui to finish it.

But I abandoned it, because with sound on, it tended to be a bit slow, and I could not fix that issue (and the sound was not really cristal clear). Without sound, the speed is fine though.

So if someone can find a way to get better sound performance...