Scratch

Technically, there's already a way to do that (on Windows), but it doesn't have anything to do with Scratch, and it requires some extra software. You can turn Scratch projects (if they repeat infinitely) by screen-capturing some of the project with a screen-recorder. There are a ton of those out there, but I'd suggest CamStudio. It's simple and free. Then I read about this program that turns audio-visual interleaving (AVI) files into screensavers, called AVI Screensaver 4.2. I don't know if it's good or not, though. You could then convert the video you recorded into a screensaver. But I suppose it would be much simpler to have a feature in Scratch that allows you to do that, but I don't see it ever happening. I think that's straying too far from the point of Scratch.