You might pick up some good advice at http://hammockforums.net. Cold weather camping is supposed to be better in a hammock, but regardless, keeping warm in a hammock and a cot has a lot in common. Especially the problem of your sleeping bag insulation being compressed underneath you by your own weight.

There is a down air mattress from a company called Exped, which might be worth investigating, and there is a cheap closed-cell foam pad from WalMart that keeps cropping up.

HTH,

A