As long as the mouthpiece/bowl never becomes so hot that you can't touch it for a full 10 seconds, I can't see the PVC causing any harm. I still advise against it because even moderate heat breaks down plastics over time, and PVC is a particularly toxic plastic. a piece of aluminum piping (or whatever, but not copper) would be superior.

Also, believe it or not, if something's advertised as anti-rust, it's either made of a metal that never rusts anyway, or it's treated with something to inhibit rust. if the latter is the case you do NOT want to be holding a flame to it.