When I first got into Due South, someone (I think it was misspamela) ran a Get Frannie Laid challenge. That doesn't answer any of your questions, but it did make me think of it. (I wrote Frannie/Stella for it.)

I suspect it's just that there's so little femslash at all, that what there is is mostly about the most obvious pairings/characters.

ETA: I just thought of this: maybe people dislike the canonical one-sided Frannie/Fraser and write femslash as a reaction?

And this. :-)