Normally, for non-edge pixels you don't shade per sample, even with custom resolve (I'd think).
Run an edge detect so you only shade the multi-sampled edges... On console you're going to be working with the entire frame with stored multisamples or you don't. You run the lighting on the entire frame (2560x720 or 1280x720). This isn't PC land with better features in dx10.1+.
In the end, anything is going to be hilariously more costly than just a comparison and reprojection of the previous frame. edit: at worst you'll get the "ghost" effect, but it really depends on the implementation and the type of game where it can be a pretty solid trade-off. e.g. large scale environments where objects in the distance don't change position much, setting a velocity threshold that disables the frame blend for faster objects, applying it for objects in the distance. Also keep in mind that the majority of folks will be playing on SDTVs or LCD TVs vs computer monitors, and certainly not in a screenshot.