I got it without really trying. I did the scene where you meet her in the village. Then I just kept doing missions for Fujimori, but also I would stop and talk to her each time. I'd tell her she was privileged, that I would be her pawn. Basically whatever she wanted, I would pick complimenting answers. Eventually I got an event where a servant was at my home and asked me to meet her at night. Then she asked me to kill Shuzen for her.
Goddamnit, it's still not working. I did exactly that and nothing's happening. --- Complaining about the graphics in Way of the Samurai is akin to hating on Tetris for its story Gameplay > gutted features for graphics
Do what the strategy guide says to do for Ending 15, but note you have to do a 100 man battle at some point (piss off any of the factions then go to that vendor) then talk to her about who would be a better ruler then Shuzen and choose the third choice.