Weather has forced me in the house too with my leaf casting and other projects with crete. I wondered if I could wrap them in damp old towels, enclose them in garbage bags, sealed, and set them over floor heat vents. Moisture and warmth...but would it work. Currently I have leaves curing in water in a plastic bin in a bathtub.

I have experience of curing concrete, not hypertufa, but I assume the same temperatures apply. It is normally recommended to keep concrete above 50F, but it cures perfectly well at 40F, just more slowly. Contractors don't like anything below 50F because it slows them up too much! Don't take any risk of freezing, that is just asking for trouble.

If you have the time I would avoid using any form of extra heat. The slower the curing, the stronger the eventual result. Heating runs the risk of uneven curing or drying, both bad things. Whatever temperature you end up at, just make sure it stays continually moist or wet.