Dennis,
Just thinking out loud here, but how about burying the whole tree and the pot sideways in a trench at the "end" of the growing season and covering with soil until spring. Maybe keeping the tree in the pot will prevent most of the roots from burroughing into the soil, thus resulting in less root trauma come spring time. Just dig the whole thing up and gently hose everything off and your ready to go. I have never tried this, but at the end of the next growing season I am not going to be able to fit all my potted fig trees in my tool shed or in my garage so I'm thinking maybe your idea might be a possible solution to the winter storage problem.