I think it is an excellent idea. Marg voiced a lot of my ideas.
Some of what Shari said reminds me of the lead character in the JD Robb (aka Nora Roberts) In Death series of novels. Lt. Eve Dallas hates small talk, etc.... Is pretty much a no-nonsense person but when she calls her husband she has to ask his assistant how she is, etc.... In some chapters she calls it "doing the dance" and sees clearly that it has to be done to get what she wants. Not just with the assistant, with a lot of different people/situation. I think that maybe teaching difficult child that this is a PITA that has to be done to get what you want may be the real result, but if it gets the results, then it is worth it.
I am NOT saying to let difficult child read these books. Not sure she would pick up on it. But it brought it to mind.
I think the jar may end up teaching her that to get what she wants with a minimum of grief she has to play the game, do the dance. Not fun, but part of life that has to be dealt with. And if it actually teaches empathy that is great. Either way, it will make her easier to live with, Know what I mean??