I would recommend that you personally not do much more than what's needed for every female, as the case may be, but leave this in particular to the individual female who wants the job, because it's HER who will get the reward or the fail, if that makes sense. You'd certainly need to assist, in almost every case, particularly if these females are inexperienced, as the case may be.
But, more importantly, if the dog isn't fucking YOU (as in your are trying to train the dog ON YOU), then your EFFECTIVE input is going to be limited by that to a large degree, again, hopefully that makes sense.
I would recommend letting the process START, as it normally would, but NOT allowing him to knot. And then getting him OFF her and having her reposition to the desired way, THEN bringing the dog back and letting him go there, all while letting the female handle her end and you handle yours, which should really consist of applying the muscle needed to both remove the dog at the initial start point, and then keeping him in check while she shows him where to put that thing now in this new position.
That should provide an immediate connection in his head if he was already in fuck mode, and now you offer him a new direction. In a way, training for a new position is quite a bit like training initially anyway, it's all about the association in the dog's head about what's gotta happen.
That's my 2 cents. And the way I would try to go about it. I've never really had this exact issue, I usually have more issues keeping him there because his attention span is squirrel short. It's trainable, but you have to keep him in place until he gets the memo.
