Steve, I think this method should work well for you. A lot of Italian immigrants in the NE protected figs in this fashion, even burying them in trenches as shown in the video posted by @TNHunter . As resilient as figs are, I doubt it will much mind the root pruning you gave it—and perhaps it will actually encourage earlier fruiting next season.
I wonder how training to a very low cordon and mounding material—maybe even soil—over this during the winter would work in 3b/4a? I’m going to experiment with this a little in coming seasons. Alternately, you could build some sort of low insulating structure over the pruned-back cordon to take full advantage of the heat coming from the house—maybe even put some sort of bulb(s) in there to provide a little warmth when things go really polar.
My own current method for 6b wouldn’t cut it in Maine. I cut/bend the figs back to 3’-4’, enclose in a wire cage, then stuff this tightly with straw and top with a tarp. Here’s a “Papa John” I’m working on now:
I’ve allowed this one to get a bit out of hand, so it needed some pretty severe pruning. When finished, I end up with something like this (though my cages keep getting bigger!):
Seems to work okay most years with most figs—and no rodent problems thus far (I put in moth balls, which I hope discourages them). If I had to do it all over again, though, I probably would’ve attempted training these figs to a low cordon, as maintaining a bush form in this way is a bit of a pain with multiple figs.
Good luck this winter!