Moving up the pads on canti brakes 12.5mm will certainly reduce braking performance due to the greatly reduced mechanical advantage. Perhaps some long reach caliper brakes?
For fun I once ran 650
c wheels on a MTB with the stock cantis but that was only a 6mm difference. After deciding I didn't like 23mm tires on a MTB I stripped the wheelset down for the hubs as originally planned
Seriously though, I see absolutely nothing to be gained by running 12mm larger rims. There is a good selection of 26" tires already, the slightly larger rims won't change anything but your braking performance, if you can get it to work at all.