Could be that some sort of activity (similar to exercising, except mental) can raise this "willpower" while succumbing to feeders lowers it. On top of that, the tricking feeders thing could be part of the feeder upgrade rework you mentioned.
You could even have this play into convincing girls to eat instead of having that be mostly based on affection+upgrades. I've always liked the idea of having to actively get better at convincing the girls to eat instead of it just being based on affection. Could be based on a combination of affection, player's willpower and the girl's own willpower instead.