look at every vintage Fender or Musicman with a maple fretboard. They are all dirty because the finish have been destroyed by the strings, sweat and maybe the skin
Dirty? Sure. I have almost all maple boards. Because of sweat and maybe skin? Sure. That doesn't mean the strings were rubbing on the finish, though.
If the strings were rubbing on the finish, then, yeah, the finish is going to wear away.
If it's just dirt from fingers, sweat, etc., it will clean off and shouldn't damage the finish - as long as it's not left on there too long.
I have not personally seen a maple board (or any other kind of board, actually), where the finish between the frets was worn down by strings (on a fretted bass).
So, to bring it full circle, I think a maple board with some kind of finish on it would actually hold up better than unfinished maple. Unfinished maple boards seem to let the sweat, body oils, etc. get into the wood and can permanently discolor it. A good finish will actually protect the wood from that. In which case, I don't see how it could be a problem for the finish to have a color in it versus being clear.