I would say just stress them to the point of herm, then let them self-seed. They'll be as fem as they can be! And the stuff about getting seeds that go herm is only partially true. If you have a plant that goes herm every time a fairy farts on it then the seeds will do the same. But if it takes a lot of screwing around with the light schedule at night, then it would take the same kind of abuse for the seeds to go herm. Well, there could be some variation, because there are still two sets of chromosomes that combine, but it would be a lot closer than from two different plants. But only the likelihood of herming gets passed down, not actual herm characteristics.