Its not being used as a raceway, its just another type of "fill dirt" eh? I suppose someone could use any sort of non-metallic pipe to act as a sleeve.
Trouble is, how would anyone who dug this up know that it was electrical? It might just look like some burried trash.
300.5 talks about backfill materials and also: "protection shall be provided in the form of..suitable running boards, suitable sleeves, or other approved means." This sounds like approval is needed by your AHJ. Though using plastic conduit would appear to be a suitable "sleeve", it might just pose a hazard for those who don't realize there is live wiring in it...but then again, any "running boards" would be just as misleading. The code doesn't stipulate warning ribbons above underground branch circuits -- just service conductors(300.5(D)(3)). If its residential, and not burried deeper than 12 inches (assuming ,120V and gfci protected), then warning ribbon wouldn't be located above the wiring shallow enough to suitably warn any inadvertant digging up of the wiring. So warning ribbon wouldn't do more than serve as a way to identify the burried "sleeve and wiring" as a potential hazard. I don't think I would be comfortable in calling that shot if I was the AHJ.