If you are going to ruin the paint you may as well take the level of the repair to the point where is will stand a good chance of lasting a long time. I seriously doubt you'l be able to get good results with a tigged repair for two reasons........first is that it looks as if the crack runs around to the monostay and welding in that area will of course melt whatever brazing is hold the mono on. Couple that with the fact that getting the crack perfectly clean so that you can get a solid tig bead in there is near impossible so it's likely that the contamination will seriously compromise the weld.

I'd silver braze a patch over the crack. If you get the surface clean you'll get a good adherence of the patch and the size of the patch will move the stresses away from the problem area. One can take a scrap piece of tube and hack it into a pair of sections that will for the most part wrap all the way around the seat tube. If the patch has a relief cut into it you'll be able to butt is right up against the mono and really have a solid repair.

You are of course right - one should replace the tube to do it 'right'. But if you need to do a short term field repair a patch will get you on the road again until you can have it fixed for real.

dave