"Holding a pad to the underside of the hammock" is exactly the premise behind the HHSS (Supershelter).
The supershelter works by having an 'outer shell' (the undercover) attached to the hammock. Inside that is an OCF pad, which also attaches to the hammock and is held up againt the underside of the hammock.
For added insulation and to act as a vapor barrier, it's recommended (strongly by most people) that you spread out an emergency mylar blanket on top of the pad, between it and the hammock.
Some people have had great success with this setup (I've used it down into the 30s with no problems at all) and others have not.
Just thought I'd mention it, as it seems to be along the lines of what you're describing.