Overwintering Lavender In Pots In Zone 5

I'm looking into getting set up for overwintering Munstead lavender for the winter of 2010/11. I expect to have several hundreds of seedlings, in 4 inch pots, some potentially bigger. My goal is to have a head start on the following summer when I will likely sell these plants after they have been well established. I've heard of the possibility of storing them, for example in a basement, or in a makeshift outdoor shelter, covered. Any thoughts on how best to do this, and/or resources to review. We are in Upstate NY, USA, Zone 5.

Munstead lavender is hardy to zone 4, but if in pots, the roots would experience as much cold as in ground in zone 3 and the change would come much quicker than for an in-ground plant. So you have to somehow prevent the temperature around the pots to drop much below freezing. Plunging the pots into a sand bed in a frame would be best, but is probably not practical for several hundred pots!
A basement would be fine if you can keep the temperature just around freezing. If it is too warm, say above 40F (4-5C) then the plants will start to grow and become very weak and etiolated.
An outside shed might work. Cover the plants with a foot high layer of loose straw - after it has become consistently below freezing. If you do it any earlier, mice will take up residence in the shed and ruin lots of plants. When the cold part of the year is over keep the door or a window open to avoid premature sprouting of the plants and subsequent killing by late frosts.
In any case make sure the pots are barely moist as they go into dormancy, as lavender rots very easily during cool conditions.