I always have a down jacket with me, either a very light down liner in the summer, or a heavier, warmer down jacket in colder weather, and have often thought it would be the perfect answer for use as bottom insulation under my lower legs and feet, allowing me to use a 3/4 underquilt. I envision something made of netting and a bit of elastic to be used to hold a jacket up against the bottom of the hammock, like a mini UQ. Even in the case shown in your photo, it would seem more efficient to keep all of the down jacket under the hammock where it is needed, since your TQ provides the needed top insulation to your feet.
I am not a DIY gear maker so don't really know how easy or difficult something like this would be, but to my uninformed mind it seems like it would be rather easy. Judging by the plethora of ideas and innovations I see here at HF, I'm guessing somebody has probably tried this and knows why it is not practical, but I haven't come across the reason.