If the split is at the front of the headtube, why not just braze a strap around the front?
Possibly the best thing to do is grind back the entire front face of the lug, then fit a patch section around the outside for say the bottom 15mm. You will doubtless also have the ability to fillet around the repair section to make it look nice!
that operation is too fiddly and would require too many areas to selectively heat. i think you'd have the silver solder run out of everwhere too quickly by by the time you had finished.
An external ring would be better because it doesn't rely on the solder for structural integrity
A ring is intrinically strong .
This part of the frame takes almost as much stress as the BB I reckon.
I'm going to go with the internal solution and actually use a steel headset cup and braze it into the bottom of the head tube. That way nothing will show on the outside and it shouldn't damage any adjacemt finish on the other tubes hopefully.
I'll take some pics and post them