Ultimately, it comes down to either an engineering decision or an administrative one.
Whether a material is adequate for the job is, I would hope, the primary question; "code compliance" is silly if the final effort is a failure.
I've been in a number of situations where it simply wasn't possible to 'meet code' and still have things work. These situations have included 'green' insulation on 'hot wires,' use of steel wire as a conductor, and the use of plumbing pipe as conduit. Mind you, in each of these instances there were details that made the choice the only physically possible choice - I'm not talking about simple convenience.
Perhaps the best example was the use of plastic water pipe as conduit -in the days before there was any listed product - for extremely corrosive conditions. That particular jurisdiction still, to this day, does not allow plastic pipe for either plumbing or electric, but was willing to accept its' use where corrosive conditions justified the use.
Getting closer to the OP, I think we've all used 'non-listed' materials for direct bore .... every time we've passed wires under a sidewalk or driveway. Naturally, my saying this opens up the debate as to 'what constitutes direct boring?" All I can say is that I was using a water pipe with a nozzle on it to drill holes through dirt long before special tool sets were sold, then passing some form of pipe through the hole. If anyone expects me to purchase a 1000 ft. reel of HDPE for a 30" sidewalk job ... keep dreaming.
"Listing and labelling" has its' uses; the presence of a UL label can make it easier to judge whether a product is suitable. That's no comfort, though, for the 'fringe' applications where the circumstances have not been encountered enough for someone to think there's a market to serve.
