I've been using a jerven bag [exlcusive with the heavy weight insulation] wrapped around me entirely, with the hammock passing through, it has worked quite well so far but haven't used it in real cold yet, not into the minuses anyway. I also have a 3/4 down underquilt which is very light 600g IIRC an packs down to a grapefruit size with a little effort which I would use in addition in colder temps. I made a full size quilt from a standard single size down bedquilt - just a couple of tabs on the corners with a length of shock cord attached by a crab to the hammock suspension - good, cheap [I already had the quilt which was going to get thrown away] but bulky and heavy compared to purpose built walking gear.
So for real cold I'd use my Jerven, with the 3/4 down underquilt and another light weight sleeping bag inside all that opened out and used as a top quilt. [plus hat, thick jumper, neck thing / balaclava, fingerless gloves]
I find it just as much a faff putting a tent up compared to stringing up a hammock an associated quilts.
With my Jerven I also have the option of a bivi if there are no trees.