Called my old master builder about this one... the old man is 82 y/o so pretty much have seen it all in 60+ years in the bussines, he is retired now.
This reminds me old miyata pro fix, those frames cracked in the same place than this one, well even lower. This is the fix and hope you find somebody able to do it... u need a pretty skilled guy with the torch... he said... clean, put a sleeve inside from the BB in, then heat and braze it from the outside, the bronze will go wherever u put the heat so thats why he mentioned like 3 times that if the guy is good with the torch it should be a pretty fast fix. He mentioned too about going from the inside with the bronze rod and heating from the outside just to play safe.
Hope this helps and hope you find a good builder to get your frame fixed, who ever it is I would recomend to add more filler in the area to re inforce it. Not a builder here ok?
Good luck.