I should say that I have no qualms with forcing a wider-spaced rear wheel into the rear triangle, where I expect both sides to flex near-equally!

It's when you do the cold-setting that one side always seems to have a lower "yield point" of force, where if they see equal and opposite force then one side springs back while the other side doesn't. Only in this scenario (what I thought you had earlier described) will a significant to-one-side offset likely result at the rear wheel.

Sometimes such an offset (toward the driveside) of the rear frame can be useful, as when a badly-bent wheel can better be trued with less dish applied, so the assembled bike could then possibly have both a stronger, straighter wheel and perfect wheel tracking at the same time! The chainline might also even improve! And, as I think you implied, what better bike to do it on than one with malleable steel chainstays?