It takes a lot of guts and willpower to step into fire by your own will.
So it makes sense that if you enter the Tower of Flames and take damage from fire there, that it can train your Willpower proportional to the damage taken. Maybe the training should also be inversely proportional to the amount of health you have left - if you are at 10 HP left, you train a lot, but if you are at full HP, you don't at all. At higher levels of Willpower, you have to also be higher into the tower to keep training.

In exchange, make morgia roots train not Willpower, but Appearance: vitamins are good for your skin and give healthy look. It doesn't make much sense that eating tasty minty herb should train your willpower.( Maybe if it was one of the damaging, vomiting or diseasing herbs, or something extremely nasty like cod liver oil)...

Pros: people get ability to train Appearance, removes scumming herbs for Willpower, incorporating it into gameplay instead; provides guaranteed method of Willpower training (no need to rely on RNG generator to create the right herbs) for everyone.
Cons: less useful herbalism skill, Willpower training will come later than normally, Willpower training will be available to everyone instead of only the characters with Herbalism.