Yvain's Doing Your Good Deed For The Day included a related study that actually got through peer review and connected it to a pastor who wrote about how it helped him understand his flock better.

Also, this works for me as an example of the kind of thing that I think LW excels at: processing criticism into reparative impulses that might work, while keeping more factors in mind than a single person's criticism/action/outcome cycle is likely to handle well. I think its easy to think "I'm doing something bad, I must change!" and then you leap into something not so well considered and it still doesn't work so well. Having this sort of content in our "canon" probably helps avoid some of the dumber things we might otherwise have done in an attempt to self-improve.

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).

I suspect that the strength of the rebound effect has a lot to do with the motivations for an action-- is a person supporting something they have an emotional attachment to, or are they fulfilling a virtue checklist?

Somewhere on LW/OB, we have discussed a study or two specifically about strengthening willpower, said studies explicitly invoking the muscle analogy and IIRC one of the willpower tests being a grip fatigue one.