EDIT: Okay, using an entire image as a texture which an image references, allowing you to do pseudo-3D texturing on a 2D pixel sprite is pretty sick, I gotta admit
What I'd really love is a script/shader/plugin that lets me do this within the engine - separate out the sprite and its texture so I can swap them out on the fly. Shouldn't be too painful to implement but it's not something I've seen anywhere before