Steve, why don't I just start from the beginning, and give you ONE approved method?
Our pipe is NOT water-tight; even if it were, water is sure to enter, as condensation if nothing else...and then, where will it go?
Burying NM is a 'no-no.' Sure, you could use a length of UF for this run.
If you're going to run pipe, then why not just run the individual wires? The pipe ought to be connected to the boxes at either end anyway- what you're doing isn't a simple "sleeve" for protection, it's more than that.
The boxes must remain accessible, and not covered by drywall.
There is an idea floating around that placing NM inside a pipe is somehow 'better,' giving better 'protection.' This really isn't so. You might even create a heat dissipation issue. And it sure is a lot of work getting NM through the pipe in the first place- which negates the one advantage to pipe; you can replace or add conductors later.
Plactic boxes, of the sort used in homes, are not suitable for attaching pipe. For these two boxes, I suggest the traditional metal box with mud-ring. You will need to run a ground wire, and tie it in with the other grounds, as well as tie it to the boxes at either end. Over this you can either place a blank cover, or add a receptacle.
A final note....Though code allows pipe to be in, or immediately under a slab, I reccomend burying it a couple inches below the slab- and covering it with a couple inches of dirt!. I suggest this, as it is likely that at some point the plumber will cut into the slab; when that happens, he will cut your wires, unless there is some small clearance between the pipe and the slab.
