I think much of the current trend in hammock design has as much to with sticker shock as good design. That and the fact that the cottage makers a biased toward sewing cloth hammock bodies vs something more like a quilt. Is is analogous to tent makers getting into sleeping bag manufacturing.
That aside, I would build a hammock pretty much like a Hennessy with a diagonal band of insulation on the bottom with a pocket between to add a pad or other insulation. I would prefer to use a top quilt or conventional mummy bag for the top side insulation. That would allow good ventilation and multi-season tweaks.
My fear with the Insulhammock design is that there is too much space inside to heat up and the full enclosure would trap too much moisture. Also, I think you can get by without insulating the full length of the hammock. That is so much more fabric and fill which equals more weight, bulk and expense.
When looking at hammock designs, keep in mind that a large part of the market is camping oriented vs hiking.
The rub with free-hanging underquilts is shifting and complexity. A hammock setup can get to be a real spider web of lines and sewing the insulation to the hammock body takes care of both issues. It would take the base price to equal a quilt plus a hammock, but wish as you might, you are going nowhere in North America with a hammock that doesn't have some sort of bottom insulation.
