Why can't the TSA make me a suit? They already have the measurements. I'm barely half joking here. Seems like this is one of those unsolved "solved problems".
You may have something here. You wouldn't even need millimeter waves to do it. Just a 3D scanner setup, which can amount to an old projector and some webcams. We should be able to use technology from 3D motion capture to build a physical model of someone's skeleto-muscular system. The hard part would be the domain knowledge. Someone would have to work closely with a tailor to convert his/her specialized knowledge about fabrics and seams and how clothing hangs off the body into algorithms. (And even then, you'd probably need a trained operator with a sense of design.)
X-rays used to be very popular for shoe fitting. I strongly suspect that millimeter waves have similar long term health issues.
And I don't see many getting naked in front of a 3D video camera set-up.
My general sense: any health effects are likely to be weak at levels typically encountered, but I'm holding as plausible that thee may be some effects.
One wouldn't have to be naked. With the right kind of camera and lighting, customers could also be wearing an opaque gown. (Some care would have to be taken to ensure that the software isn't tampered with.)