What you are suggesting is to bake the entire frame, lugs, solder and tubes at the same time?

I would seem to me that you would lose control over the brazing process by doing that while gaining nothing. And the frame would not be able to be worked under those conditions, so adding more solder wouldn't be possible once firing was underway.

I really don't see an advantage with that, even in time savings.

What I would be curious to know is whether a reliable lug could be made in a sintering oven using powder metal technologies. Sophisticated sintermetal technologies exist already that are able to impart varying qualities (hardness, rust resistance, etc) of metal to different parts of a single sintered piece by varying the content of the component metals, specifically in stainless steel applications.