The problem is that it's agnatic-cognatic instead of absolute cognatic. Which, if I understand right, means a woman can't be nominated as long as there are any male candidates in my dynasty (or possibly in the realm at all). Since I'm not playing ironman for once, as it's pointless with a mod (particularly the looong autosaves), I might do some console testing to see how that actually works.
To pull this off I'm going to need a ruler to only have daughters, and for the succession to be primogeniture (or ultimogeniture or gavelkind). Otherwise there's just too many candidates for agnatic-cognatic to be any different from agnatic, as far as I know.
I never liked killing dynasty members even under the most crucial circumstances, but I think it's literally impossible to plot against sons now? And the assassins are gone too. So I just need to have a daughter then get a divorce I guess. Or maybe find a mod which provides contraceptives... That seems perfectly reasonable honestly, particularly with a failure chance. As-is, even a gay ruler will have as many children as possible, even under gavelkind.
Heck, I bet there's a mod that lets you claim children are bastards. Preferably with a big prestige hit and a lot of ill-will. At high crown laws you ought to be able to disinherit someone, at least as part of banishment. They'd keep a strong claim of course. I tried that this game, the banished son went to an enemy court but remained my primary heir.
I guess the only vanilla options now are imprison-execute or getting them killed in battle... Both of which require the kid to grow up, and die, which I don't want.