here is a thread with some discussion of the subject.
http://www.bikeforums.net/showthread...liper-location
But before you get too far....what are you doing for hubs?
AFAIK Disk brakes require hubs designed to mount the rotors. Does any one make these in a 120 mm hub? If not you will need to solve how to reset dropout size (not easy on aluminum frames) or find an adapter, which might mean adjusting wheel dish. so you need to take hubs in to consideration also
Nice renderings by the way.