Has anyone heard or considered a hammock that has a stricture at the place where a neck would be?
It would need two connected spreader bars to prevent independent rotation but you would be able to have a 360 degree sleeping bag that cinches tight around your neck. That way you need no separate under quilt.
Of course, of it is heavier than a top and bottom quilt then it's not worthwhile.
It might be difficult to make it comfortable though.