I overwintered a tuberous begonia once in my cool dark pump room. I just stuck it in there just before the first really hard frost, pot and all. Brought it out the end of February and put the pot in my sunniest window. I gave it a tiny bit of water. it took a couple weeks but it started growing back. I'm not sure how the foliage would be coming out of the dark as the one I had broke right at the soil level just before I put it in. But I imagine you could just cut back the foliage before or what's left of it after you bring it out.
It was a blast looking for new growth!
