Thanks for replying, Tim.
An inflated underquilt would need to be rather slack and very much a differential/shaped cut. My though it is that it should be synthetic fill like current air pads and incorporate a reflective surface on the hammock side. It could be far lighter fabric than any air mattress and should be full length. If Therm-a-rest can get R4.9 out of a 2.5" think pad, a 3"-4" pad with a similar R-value should be approachable by a cottage manufacturer. The inflation would be just enough to get full loft and it is hanging down, so gravity would aid the process where it deters it on a top quilt. All suspension/stress should be on the top/hammock surface.
If a full-length suspension layer is used, the last 12" or so on each end could be just fabric to provide wind blocking and simplify the suspension. The sides could have Velcro, snaps, or zippers to hold them to the hammock. The resulting space between could be used to add more insulation, like Insultex or Thinsulate.
I think making universalized components as the end project would be a mistake. To eliminate the Rube Goldberg arrangements and reduce weight, the hammock and insulation must be designed as a tightly integrated system. The current state of the market is a little like selling poled tents and leaving the buyer to go get a rainfly that kind of fits. Fortunately, the topside insulation and the tarp designs can be much more universal yet remain light and effective.
I've also thought of making a hammock that has a deep outer pocket with crosswise drawstrings to control the space and insulated pads could be made to go inside. The pocket would have Velcro or zippered access on each side and some means of attaching the pad to suspend it. The stress would all be on the hammock bottom. Insulation could be down, synthetic fill, reflective/space blanket material, or any combination. My preference would be to use a space blanket in conjunction with synthetic filled pads. The outer covers on the pads could be the lightest material available, as they would only need to protect the insulation for packing and laundering.
This is really just taking the Garlington Insulator undercover-and-insulation concept and integrating into the hammock rather than a separate item. Clark accomplishes the same thing by adding a series of pockets down the side of the hammock. Six pads and end insulators make up their Z-liner insulation system. See http://www.youtube.com/watch?v=uaGZHrKrK50 for a user-made video showing the Clark Z-liner.
I find it indicative of the market that Clark doesn't tout the need for insulation and you must hunt through their web site to find that they offer the Z-liner kit-- it is buried in the online order page. The Z-liner kit nor the need for insulation is not mentioned in the specs and description for the North American model, which only describes the "insulating pockets."
I don't know of any hammock manufacturer's web page that tells the prospective buyer that a hammock is useless for sleeping in sub-70F weather without some form of bottom insulation. Take a major retailer like REI, where you will find several makes and models of hammocks, but there is nothing mentioned or even offered for bottom insulation. I'm sure it is an oversight rather than some dastardly marketing plot to rope in the buyer. For example, you can drop $150 for a Hennessy Explorer which appears to be a nicely coordinated system, only to find that your 20" ground pad is uncomfortable and not wide enough and you need to add a $130 SuperShelter which weighs a full pound and adds much complexity to the setup. You won't find that mentioned on the web site-- you need to discover it yourself by trial and error, or find in mentioned in a forum. Once you know to look for insulation components, then you will find the product and video links. I don't aim to pick on Hennessy exclusively, as ALL the cottage and factory manufacturers are guilty of the same.
We are looking at a whole outdoor equipment category that is relatively young and in a constant state of development. I would liken it to the development of external frame packs where things kept getting bigger and heavier until there were design and consumer revolts.