Frost Free Silcock - At Grade - Problem in Winter?
The frost free or freeze proof part keeps it from freezing outside the house envelope. If the pipes in the crawl space get below freezing for a few days they could freeze but I would not be worried about the Silcock as snow actually acts like insulation.
 Ever heard about people sleeping in snow caves? I have done it at -20 and been quite warm. 
It sounds like you have the crawl space well insulated and heated so I would not be two concerned about the Silcock freezing at all.