Ah thanks for chiming in with experiences from the field. I think it is clear then, camping in a (Hennessey) hammock in the cold is not crazy it is just fine. I suspect it may even be warmer than in a tent (with a bit of padding under your back) my reasons:
The nearer you get to the ground the colder it is, when i am lying in a tent and it is cold out i can really feel the cold being drawn up into my body/radiating upward from the soil. The Hammocks interior is smaller which may (not sure about this) mean that your own body heat will have more effect and reduce the cold in comparison with a tent.