Cycles Render Engine UV render pass node and scene setup
Video Tutorial http://www.screenr.com/lT9s
The new image should be applied via the UV map to the diffuse color pass. That way,( if the cube had some gloss and a bit more geometry ), it would look like the texture was applied on the material before rendering. You have to play with the direct and indirect pass as well, because there are reflections of the original material in the scene.
Thanks, man! How can I improve the antialiasing of the texture applied?
It looks like it could be interesting and useful, but after looking at the .blend file and viewing the silent video, I still have no idea how this works, or why it would be advantageous to use this method instead of applying the image to the cube in the usual way.