Possible to animate (move) an image UNDER a mask?

What you need is to do this old-school -- create a background graphic with a transparent "hole" in it, and put your image behind that. Most graphics apps, such as Photoshop, Graphic Converter, or any of the more recent modestly-priced apps can create this kind of graphic.

I imagine it would be a big challenge from a usability perspective to have an object mask that could be in a completely separate location from its object. Perhaps a more elegant solution would be to allow for masks that punch holes in objects -- this I think would be easier to work with, and give the same effect when used on an overlaying graphic (such as a background).