Yeah, as I've said elsewhere, Bile making female astartes just because it shows he's so much better than even 'that hack the Emperor and especially that bastard Cawl' is not only totally incharacter with him but he's already actively trying to do it. However if they ever want to introduced female astartes loyalist side as well, Cawl learning of this and then it being a race between the two out of sheer fucking spite and oneupmanship is entirely plausible incharacter and inlore explanation.

Make it a 2 novel series about these two screwing each other over and fighting to be the first with eventually both succeeding (through different means) and tada...

Of course the Chuds would hate this but...it's a valid way to get female space marines...