I did a search of the archives, and didn't find any good solutions, but it seems to be a fairly common problem.
I've measured the frame, and fortunately it is factory straight. I've cleaned out the inside and checked it as best I can--the TR frame has a lot of vent and drain holes, so it's not too hard. No rust, fortunately; got a mouse nest out of it, though, and a bunch of pecan shells.
I considered welding in a new piece. I think I can do that successfully, without it looking like a repair. Treat it like the body, grind down the welds, and put a bit of high-build primer on it. That was the other thing I considered, but was not sure if that was sensible or not. Mostly, I don't want it to look like the frame has had a rust repair. The upside, of course, is that I could get a good look inside the frame and see how it is doing.