Very cool!
I wonder if someone more creative than me would be able to push this to do things it was not designed to do. I recently found a video where someone exploited some properties of certain transcript file formats to be able to make a primitive simple drawing app with Youtube's video player's closed captions.[0]
Since a brush's code can see the state of the canvas and draw on it, perhaps there can be a brush that does the opposite here, and instead renders a simple "video" when you hold down the mouse? Or even a simple game, like Tic-Tac-Toe.
I understand that obviously isn't the purpose of the brush programs, but I think it is an interesting challenge, just for fun.
[0] The video I am thinking of is by a channel named Firama, but they did not explain how they accomplished it. Another channel, SWEet, made their own attempt, which wasn't as full-featured as the original, but they did document how they did it.