So after stumbling through handmaiden.dlg, I've identified the string that leads to her joining up.
Unfortunately I'm at a roadblock on this, so I'm turning it over to the group at large.
(This comes from the "You may ask." line, that is found under the tree where she starts with "You fought well")
Text: Why do you look different than your sisters?
Conditional #1: String Param (000_Hand_Mom_PC)
These leads to you having the option to say:
So you have a different mother...but the same father?
Before, she refuses to discuss it, and I'm pretty sure she never will if you are female.
The entry begins at 87.
"Yes, that is correct. I feel that I may trust you with such things, so I shall speak of it, if you wish to hear it."
No matter what you take on that tree, it always comes to her saying:
"I want to fight with you side by side"
The only problem to me is sorting out the conditionals, you get two options when this comes up, after some fight between the pair of you.
I do not wish to discuss it. If there is something else you wished to ask, you may do so.
Conditional #1: c_influence_bet
That is the female only version, I believe.
Yes, that is correct.
Conditional #1: blank
I think the easy switch would just be to swap the conditionals on those two. But I'm not sure how to initiate a fight between the PC and Handmaiden. The only scrap of dialog about her challenging me, leads her to say "It's not the time."
But maybe this is enough for someone to work through it and make it so Handmaiden is open to females easily. And from there, something can be done to block off Disciple if Handmaiden joins. And have him available if she isn't.