I can gladly report that this was wildly successful. Off went his clothes, and I just plopped him in the bathtub with three separate pots of paint and paintbrushes, and it didn't take him long to make masterpieces. Between the time it took to get him ready, paint, clean the tub (which only took a washcloth and Mason did most of it himself!), and bathe him, Mason was probably in there for an hour and a half. A miracle!
Of course we (the parents) had so much fun that we had to FaceTime with all the relatives, and at one point we squeezed four adults into our tiny bathroom along with the artist himself. Yes, we might be crazy. The best part was watching his little mind work. On his own, he painted a rainbow, a bird in a nest, several monsters, and a purple banana (he narrated this, of course). With my help we painted his name, Elmo and Cookie Monster, and some various other characters. I love watching his imagination develop. I'm sure we'll be doing this again.
I just have to share part of the conversation we had on the way home that day. After I mentioned painting to him, the subject changed and I asked Mason what he wanted for dinner that night. His response? "No thanks, I'll paint!"