Sounds risky to me. I'm assuming that you will be insulating the floor. I'd consider building a chase for it right under the floor - far enough away so any nails, etc., won't be a problem, then putting the insulation over that. In that manner, it is right next to the floor, inside the insulation, it has a better chance of not freezing. If the water is flowing, it would take a really cold day to freeze it up especially if you were running hot water heat through them. Problem comes if you decide to shut the room, turn off the heat, or lose power...then it could get ugly. PEX is unlikely to split if frozen, though. Fittings, if there, can and will.