I had no idea! That's weird, huh? I'm sure, though, that I've seen FxF couples as background characters in at least one omegaverse yaoi...
They're supporting chara, parents of MC
http://www.mangago.zone/read-manga/hebi_no_seikatsu/
Yes me too, it's after reading this manga that I wondered if it was a thing or not in the yuri's universe.
It's weird but maybe not so much. I mean, for an alpha female to mate with her omega and have kid, a penetration is needed, her clitoris turns into a penis in order to do so. That's kind of... strange, since it's a lesbian relationship. I can see why mangakas who draw yuris may be troubled with the idea.
I'm curious... Is there any yuri alpha/beta/omega story ?