"I could--for the short term--do the same procedure. Wire wheel and clean off a good few feet around that area. Weld in some heavy sheet metal. Spray some internal frame coating stuff. Undercoat the hell out of it. Done. "

That's the only solution afaik. and it sounds like you've got experience doing it. Unibody construction is very strong as long as you keep it rigid, and rust repairs do just that. Good luck.