I tested the painting thing this weekend. It works as reported. And man, is it enticing. Instead of watching a sim do their thing on a large canvas it's "click paint, stop, continue, stop, continue, stop, continue, stop, continue, done!" Takes less than an hour of game time to do, I think. Feels so wrong, but works so well!
I think this might be harder to enforce on tourney challenges because of how long it takes to paint a large. Before I knew about this exploit, my sims would often stop painting anyways because of hunger, exhaustion, or the random booty call (who could blame them!) I guess the rule could be that if the sim stops for any reason, you have to scrap the painting. That could be pretty painful though!