Just got back from holiday to find the Jack's R Better Weathershield awaiting me.
It's basically a rectangle of waterproof breathable fabric, with attachment points and draw cords. I set it up last night under my HH explorer with no bottom insulation. It felt much warmer than the bare fabric of the HH. After a while the cold started to seep in so I put a 1/2 length blue foam mat in between the undercover and the hammock- perfect for spring (In Australia) temperatures.
I believe I'd be able to get away with this settup for part of the year, and my JRB Nest underquilt in the colder months.
The set also came with the overcover, which is another rectangle sewn with a footbox. The overcover slides over the quilt or bag and increases the warmth quite a bit. I have not tried it yet, but I believe it could be used as a form of half-bivi under a smaller tarp for ground use, to provide protection from slashes.