I don't wish to engage "willpower is X" metaphors any further.

Every experience I've had where I'm tempted to think "you sure have trained your willpower muscle lately" is also entirely explainable by altered self-image, consistency, and habit.

[charity is (habit-forming / refractory)]

Yes. You really have your work cut out if you want to show some sort of generalized morality or willpower training effect (as opposed to a task specific one).