Thanks for all the insights, it's very much appreciated.
I think I'm leaning towards pinning the whole frame in the jig, taking it out and brazing, despite the potential issues of alignment, and the trickier straightening after brazing. My thinking is that, if I can get 3 pins per tube end, I can not really see very much movement being possible. With 2 pins on oposing sides of a tube, I can see movement around that pin axis being possible, but not with 3 pins. I'll do another test lug with pins, as I still have some tube and lugs left over, and see how it goes. I may still change my mind depending on the test lug.