It can be done, for sure, but I'm not sure it can be done well. Brompton folding bike frames are fillet brazed steel, and they used to sell one that was just clearcoated to show off the workmanship which, I'm told, is very pretty. But they have stopped offering that finish; apparently it just did not hold up well enough. And that in a factory that prides itself on doing things right. So... if you have access to all the right equipment, I'm sure you can do it. Maybe as well as Brompton; I, for one, could not.