Does anyone know how to mount a disk brake to a cotterless of square tapper hub?
There isn’t a square taper hub that has mounts for a rotor. You will have to use a crank mounted rotor from Mountainuni.com. Along with the rotor you would also need a 110mm bcd crankset to fit the rotor. This is a common size for junior bmx bikes, and so you should be able to get a length that you like.
Of course if you can weld you might be able to modify an existing hub to work with a rotor. Personally, i liked the external crank mounted rotor i had on my Nightrider. I used a 140mm crankset, and i had a UDC CrMo hub.