Base 2 I put brazed cantilever brake posts onto a 26" Varsity frame 36 yrs ago to use in early MTB racing here in Northern California. I had no trouble with the Ashtabula fork using this system but the Mafac Tandem cantilever brakes actually started bending the seat stays forward. Smaller Varsity frames might not have that problem. The bicycle was being used as a race bicycle where conditions are much worse than for everyday riding. Racing tends to weed out the weaknesses rapidly. Varsity's are not alone in having this problem. After a while MTB frame makers put on stouter seat stays that would not bend when using very strong brakes. I love your idea of longer rear forks. Rivendell has gone to this idea and if I get enough scratch together I would love to get one of their long wheelbase bicycles.