Short answer, no, sorry. Basically you would have to work directly with other channel authors and convince them to update their channels with a bunch of new code, and each case would be unique and a lot of work and testing and no one is going to do that. But you're free to try.
You can create a sceensaver application.
A screensaver would stop working when the user interacts with it though.
But if a screensaver works can we assume that Roku probably has the ability to run two apps at the same time?