Erm, why?
If Sony controls the swap, they would control the frontbuffer, hence there should be no additional memory(aside for slightly larger FB), and no additional latency needed.
Hacking around the game processing would give the penalties you mention - but again I don't see why you'd need more then one buffer, nor would it necessarily introduce extra latency (of course the time taken out could slow the game itself down).
Granted - the latter is an incredibly ugly(and potentially unstable) way to do it - I'd sooner see them release game specific patches instead.
Assuming you want to support any rendering effect, you have to leave the frontbuffer (as the app sees it) available to the app, so it can only be a source, not the dest.
On the "swap" you would want the app to be able to continue immediately, so you'd start a parallel scale it to your "OS front buffer". I guess depending on where you block it could just add the latency cost of the scale. Although unless you can do the scale in the VBL time, the OS buffers would have to be double buffered aswell.
I guess technically you just have to chase the raster, so the scale could take the whole frame, with a single buffer.