Yup, lots more work than its worth though I reckon
Unless someone gets it super duper cheap
It would want to be something you REALLY wanted.
I’m lead to believe this is a very common problem on some titanium frames and I know that Supermanlegs had this happen to him on one of his old titanium frames. I’ll defer to Blakey on this one but my understanding is that impurities can enter into the weld site (in this instance the site of the derailleur tab), and weaken the tubing at that point. It’s just a matter of time until the tube cracks along this weak point.
You’d have to be a shit welder to let stuff get into the weld pool and contaminate it, thats what backpurging and gas lenses are made for
I wouldn’t weld a derailleur tab onto a titanium frame, much better idea to use a clamp on mount. Stresses are introduced into the tubing when it is welded and the hot metal contracts, AFAIK the tab is a problem spot and would be asking for a crack unless the whole frame is shot peened or something to reduce the stresses inherent in the weld.