Good question.
Is this one of those "tile" type floors like you have in computer rooms? I would think that the AHJ would prefer the receptacle to be mounted on or above the finish (raised) floor, for easier inspection/access.
If I was faced with this situation, I'd probably surface mount a J-box (or dedicated duplex box) to the tile, and attach that to the existing "sub" (real) floor mounted raceway system by means of MC/flex.
The closest ref. I can find is Underfloor Raceways (390).
When in doubt... ask your AHJ?!
