I think there's some 'legal amnesia' taking place. What did we use before there was HDPE? After all, HDPE is pretty new to the code. Were we not doing direct bores before then?
Just because one product is listed for a specific use, that does not mean that only that product can be used.
As for re-tasking materials from their original use and applying them to electrical uses, that's a call for the AHJ to make. While the code says 'use one of these ...,' the code also recognizes that the AHJ can waive / approve / make exceptions; that's why he's called the "AHJ."
Make your case as to why the substitution is justified, and let him earn his pay.
