It sounds like as long as the pixels are changing it doesn't matter much.
Instead of a screen saver, what about an app that runs over the top while no mouse is moving. Same idea, just not in a screen saver format. So for your bubbles, have an app that waits for no mouse or keyboard, then have a bubble go from bottom to top. Or a clock that has an adjustable size that goes from side to side, top to bottom, bottom to top, diagonally, does a little dance, etc... just to change the pixels. Wouldn't have to worry about updating the screen since it's updating, just being covered by the other app.