Well, I had planned to take my down parka in case it got really cold. But sleeping in it in a hammock is not like sleeping on a pad in the tent. The down would compress. So I am considering sacrificing it, too, along with my older Marmot, to the underquilt role. Together they would do what I envision: respective ends of the hammock could be strung through the foot of the bag and the hood of the parka to prevent warm air spilling out the ends. Their combined length is 114" of premium down insulation.
I suspect that shutting the door at the ends of the hammock is well worth the extra weight, which I had planned to carry anyway. My go-to Mountain Hardware bag will serve as the top quilt. If I truly sacrifice these items by cutting off arms, the hood on the bag, zippers, velcro tabs, cords, et al, then that will actually reduce the weight.
This will cost me nothing but some dutchware, which seems like excellent equipment to me.
The question arises what I will do for a parka during the day if it is that cold. Frankly, I never liked wearing that thing, even in Maine. It got too hot chopping wood, for instance. I always took it off anyway so will have to solve that problem as it occurs. I'm thinking wool (and alpaca for another thread). Many of these things are new to me.