As long as the sidewalls are thick enough and have a vertical surface, like a U shape, then I don't see why you can't brakes use on them. To be 100% sure you'd to see the cross section of the rim, as there's a chance the bit of the sidewall you can't see is really thin, although that's unlikely.
Machines braking surfaces are relatively new, rims didn't used to have them.
Actual track only rims have almost no sidewalls.