George,since we all seem to agree isolated sections of pipe are not the "metal water piping system" I would look at 250.104(B). That is more appropriate if you can make the case that they may become energized. Actually I bet the Insta'hot does ground the hubs where the pipes screw in, via the EGC.
I even have the compressed air piping in my garage bonded. My guess was that it would probably be a 20a circuit that "may" energize it but I went with a #8 anyway.
