Mark, I am in Dayton / Cincy area and work on crawls a lot that have no heat in them and they never freeze. The most important thing is that there are no openings to the outside. The ground heat should be enough to keep it from freezing.
I am also interested about the freezeability of PEX as I just started using it. I bought a nice set of brand new Zurn crimpers and gauge off that fameous auction site. Got a 1/2 and 3/4 (not the combo unit)for $124.00 delivered. Can't beat that.