I don't see how this ban can feasably work. There are already several polymer receivers for a wide variety of guns out there and even polymer FCGs. And guns with those features are far from undetectable.
I just don't see how 3D printing can be specifically singled out for a ban. In order for this to work they would have to completely ban either polymer parts for firearms or private manufacture of guns. Neither of which is going to happen without a hell of a lot of resistance from gun rights groups.