It will be far easier to fix an aluminum rod rack than a tubular steel rack in the middle of nowhere.
I think you are right about TIG welding aluminum rod. As long as the metal has not fatigued, you should be able to TIG weld it. As for chromoly steel tubing, I think the ability to TIG weld it depends on the tubing sidewall thickness, and I just don't know how exotic the tubing is that Tubus, Surly, and the boutique rack builders use.
As an alternative, I will bet you could find someone to braise (silver solder) a tubular steel rack. Again, as long as the metal has not fatigued, the repair should be good as new.
I have a buddy that wants to modify Tubus rear racks to better accommodate an Arkel Tailrider rack bag. Nice bag, just perfect for all the stuff you might want immediate access to during a ride. The Tailrider just wants a wider platform to sit on than what Tubus supplies. If he ever follows through with the mods, I will let you know how easy it is to TIG weld a Tubus rack.