Well, I finally had occasion to investigate this further. Spinning the wheel sans tire there is no apparent deformation or out of round. Sidewall is way too thick to be flexing. Mic'ing across the out edge of the channel shows a consistent 1.0750" ± maybe .005. But at one very short - 1.2" or so - section it flares to 1.0870". I can't help but put this down to damage, but there is no indentation or other evidence of impact, and I can recall no incident in the approximate time frame. It must have been recent because there is no evidence of additional brake wear at that point. Is a puzzlement.

I'm also surprised that a mere 12 thousandths would cause such a dramatic symptom...

I think I'll try to reduce the flare with a bench vise. Or I could try a hammer and dolly. Any other suggestions? Should I use heat?