Hi @creedon
My initial thinking is not an entire background, but an isolated element such as an 'Image' block. I'm not sure what the right idiom is.
Suppose there were an specific kind of element available — ImageAnimation — that allowed me to provide parameters such as a URL from which to retrieve the frames. When this element was in view in the browser window it would animate those frames forward as the user scrolled 'down' and the frames would animate backwards as the user scrolled 'up'. Once the last frame was reached, the browser would continue its normal scrolling down or up the page.
I don't know enough about the possibilities for customizing SS to know whether it is possible to create new elements, but generally speaking — this is the kind of effect I would like to achieve. Although I realize it may be only possible through some other fashion aside from creating a special kind of Image element.
Curious as to your thoughts — seems like you know what you're talking about..