Although I haven't tried one of these designs, that hasn't stopped me from thinking about it. The biggest challenge I see is that hammocks stretch a LOT, which has the potential to compress the insulation. Therefore, I would consider making the hammock out of the least stretchy material that you can find. Perhaps a thicker polyester, instead of nylon.
Of course, that would add some weight, so I guess the quest to develop the perfect hammock continues.