I seem to remember seeing a gas tank repair video. I think they welded it, then filled in the pinholes and whatnot with silver solder. I'll have to see if I can find it.
edit: ok, they used bronze first, then went over it with soft solder to fill the pin holes. This is part of a series of videos from KBS coatings on gas tank repairs.
Repair video (repairing starts at about 3min):
YouTube - KBS Coatings - Gas Tank Sealer - Part #2
All their videos:
Videos - Demonstrating How To Use KBS Coatings - Stop Rust