i reckon u couldn't weld it unless u had the proper jigs to realine the frame after welding
...and during.
Not an easy job with conventional lugged frames, as you have to cut/section the seat tube, then ream out the seat lug/collar and heat/remove the stub of seat tube from the BB before reversing the process. It is usually easier to detach the ends of the seatstays and top tube and braze in a new seat collar and seat tube.
Heaven help you if the builder pinned the joints.
Lotsa heat/reheating, which is generally NotGood, so I try to minimize the heat on reassembly by using silver instead of brass.
In this case, there is a bi-laminate seat lug (fillet over lug) and all the joints are (beautifully) fillet-brazed, so you'd have to melt that fillet and risk the other joints at the BB below and the seat lug above (seatstay ends and caps). Then, more heat with new fillets flowed and...no. Not worth it, especially adding the cost of a respray.
a fella would want to be out of his mind to part with money for it.
<nods> If any other tube had broken, it would have been an easier fix.
I wouldn't want to try it. I'm still shaking a bit after my last successful go on a now vintage Bridgestone RB-1.
Best,
Dan.