The reason MTB don't use them is primarily for mud clearance and tire clearance. You would have to make them extremely large to fit larger tires and to allow mud clearance on top fo that. Even Cyclocross bikes dont' use sidepull brakes for those reasons. Technicaly would be possible, but to make them that large ther would be a a big weight penalty and I would think woudl get clogged up with dirt/mud alot easier.