Walrus, that's kind of the dangerous ground I'm walking on, in suggesting the use - in certain circumstances - of plumbing pipe.
First of all - as a reminder - the AHJ has the authority to approve alternate materials. Article 90 and all that.
Secondly, in the case of pipe .... "listed" seems to be, in this case, a distinction without a difference. (I can hear the howls already!) That is, pipe is a pretty straightforward product, and the ASTM specs for the two products ensure that the similarities are more than skin-deep.
After all, what's there for UL to inspect? Look inside, and say 'yup, it's smooth?'
I've been down this road - the use of plumbing pipe - a few times before. While this was (in the past) on account of the materials needed for chemical resistance, and was run past the AHJ for approval, the principles apply here.
Indeed, I've also had a job where rectangular, structural steel tubing was used as a raceway ... with AHJ approval. It was simply a solution to a wire protection problem. The point, again, is that there IS room for innovation.
Not that any of this is to taken as a wholesale disregard for the usual practices. I've yet to make a mast, myself, from anything but listed RMC. I have no plans to do so, either. If I had a mast that needed to project above a roof more than 7 ft., and my run to the meter below the eave was more than 5 ft., I'd surely consider it, though - along with substantial additional bracing!
