There is someone around here or MTBR.com that has a mtn. frame designed for 26" wheels and has 700c wheels working with cantilever brakes.
As far as the wheel and tire fitting, that shouldn't be an issue. The issue is the brake pads reaching the braking surface of the wheel. Tektro has cantilever brakes that have cylindrical arms where the pads can be adjusted up and down over a very long range. I found a pair on eBay for $19 new. I have not pieced my rig together yet (converting a circa 1990 Schwinn High Plains to a cyclocross/commuter bike) but a test fit with a 700c wheel with the cantis in place appeared to work. Hopefully by the end of this week I can tell you if it will work well or not, but I'm following the lead of someone who already did this and it works for him.