Best answer seems to be "it depends.". I had a broken leg a couple of months ago and went thru several checkpoints with a boot cast and crutches. I opted out of backscatter scanners (could not stand without crutches, but would have opted out anyway).

At BOS, the cast was patted down but no one asked me to remove it...which I wouldn't have done if they had. At Columbus no one even touched the cast, although they thoroughly patted down the airport wheelchair I was using and ran the crutches thru the X-ray machine a couple of times.

I considered myself lucky -- I had fully expected to be subjected to a breast and genital grope since that's how most disabled passengers are routinely treated. I haven't flown since and will avoid doing so as long as I can because of the current violations against our freedom and human dignity taking place at airports all over the country.