I decided to continue my investigation by doing some rough camber measurements in our driveway, using a right-angle metal square and a 6" caliper - I knew these would be subject to lots of uncertainty, but it's a place to start.
Well, mystery solved! Following is the camber I measured:
Lrear -1.9deg Lfront -0.31deg
Rrear -2.5deg Rfront +0.46deg
Now, the right front wheel has a pretty scraped up rim, presumably from tangling with a curb. Pretty hard, too - I think. No sign of damage to the tire, so I think it was subsequently replaced. Removing it to inspect the hardware, I discovered clear signs the adjustable lower arm pivots, at least the rear one, had moved a considerable distance inward along the adjustment slot. I did not find any apparently damaged parts, though I suppose the bushings have been abused.
Since it looks as though just an adjustment should return all to proper alignment - or at least an alignment would be necessary anyway to determine if the frame was tweaked - this is now certainly the next step. Thanks all for the responses!