I'd give up on the cantilevers and keep the v-brakes. What kind of sifters/levers are you planning to run? I'd go with the special drop style levers made for v-brakes (Dia Comp 487s or something?) and bar-cons using friction shifting for the front deraileur (what's your cable routing? It makes more if a difference with STI shifting-- friction almost always works when making a franken-bike)
If you want to keep the canti brakes, try finding a small seatpost collar or even a reflector collar, match it with hanger syle stop, and mount it on your seatpost, just above the collar. It's pretty getto, but I think it will work.