I have been a hard core mtn biker for 3 yrs now (500-1500mi a year on single track) I have had hayes brakes on my bike and let me tell you they rocked! my bike weighted in at 30lbs but i have friends who ride down hill bikes that way up to 60lbs so i dont see why it wouldnt work. also they spin the 9" or bigger brakes and use a 20mm axle
dont see why that wouldnt work. The biggest prob i see is a way to fab up a mount that will be strong enough... they usually mount to a part of the fork. a better Q might be can you get a triple tree mtn bike fork to mount up to a moped? they use a 1.5" stem.