Thanks to all for the input. I have no pix as I took it straight from the LBS to the painter. But the shell seems otherwise entirely stable. I've had the bike a little under two years and removed the surface rust (although not under the cables) when I first got it. Then got caught in the rain a number of times this summer and the problem recurred with a lot more paint flaking off, so I decided I needed to get some paint on it. It's sufficiently superficial that I'm relatively certain the structural integrity isn't otherwise compromised.

(I had a 'that really shouldn't be possible reaction myself. But there's no indication it happened from the inside and I can't think of another explanation.)