Did you know that female mites react very sensitively to high relative humidity such that they almost never reproduce at levels above 80%? If you could keep your hive at 80% RH, it would be virtually impossible for an infestation to occur.
I believe you can most certainly use this to your advantage when overwintering and during spring buildup. The hive is a very humid place. Why do you need an upper entrance? It's to prevent all this excess humidity from condensing over the bees and dripping down onto them. Well, why not just heavily insulate the top of the hive (more so than the sides) such that all condensation forms on the sides of the hives? That's exactly what I did this year.
I decided to go without an upper entrance in order to maintain the hive at the highest humidity level possible. I could care less about condensation (and I have a lot of it). As long as the condensation forms on the walls of the hive, I figured I would be OK.
My results (so far) are fantastic. Very good orientation flights out of both of my hives yesterday (temps in the 60s). I pulled the bottom boards and could not find a single mite. Believe me, I know how to find mites.
This overwintering technique would be very similar to a hive overwintering in a tree.
Now, I believe at this time of the year my queen should begin laying in earnest for the spring build up. I'll keep you guys informed on my results.