Short answer - yes. Using this new found knowledge (which I'd never thought of), can anyone think of a way to get "Genre is only x" (i.e., no multiple genres)?
Using Genes as a virtual tag, that could.be done with regex.
Added: actually, to get "genre is NOT only x", it might be easier to use the virtual tag to do the check, with a $Match function, and then filter based on the result (I.e. Y/N as the output of the virtual tag).
I think a wishlist request to be able to use Genres in filters is in order, either way.