A greenhouse should work ok. Since basjoo produce lots of pups, it should be possible to keep smaller plants alive indoors, dry store large corms in a cool dark basement, and cover some corms outdoors. Not putting all your corms in one basket (haha) might guarantee success!
If you have that much rot, I would suspect that too much water got into the soil. Are you using amended soil or just the typical clay? I cover my plants with leaves about 1 foot beyond where the corm ends to keep water away. I do have plastic coverings that stay on until early spring. I remove them once I start to fear overheating more than deep freezes.
This spring is still cold and wet but I have a velutina in bloom and a slow moving basjoo...
erik
