guitar, if I'm understanding correctly, your shader doesn't help with what I'm trying to do. My game concept hinges on using external scene elements---scans of hand-drawn images, digital paintings, or renders from other 3D software.

Using a render from a view, I am assuming that you are using depth information from the view to determine when an entity should be 'covered' by the view image. If this were modified such that a user-entered depth were assigned to the image and the image could be loaded from bmap, that would be a start. However, there would still be no system to manage animations. Perhaps a media stream could be assigned to the bmap that is passed to the shader? Otherwise, one would have to create a system to do what sprites or media streams already do to display animation.