With steel tubes and brazing lugs are more to locate tubes and spread joint stresses then to be the strength of the joint. I have always thought that one could trim back the socket depth of a lug to 4mm's (or so) and not loose any joint strength (as long as the miters were done well). It's been said that most all lugs are overkill as far as strength goes. It's been said many times by builders far more experienced then i that the joint's integrity comes from the miter, tube to tube contact. Given this any material that is brazeable could make a lug and be OK. Enough of the material would be needed to equal the strength of a steel lug I would think.

I have joked that one could make a frame from Styrofoam and chewing gum, if enough was used. Andy.