I spent a cold an windy Sunday night inside my HH. There was a strong 25-30 kt onshore wind at the beach and the only available trees were slightly too close together, so I couldn't stretch my tarp and it billowed and flapped in the wind till I rolled it.
Luckily, the trees were more or less in line with the wind, but there was still a chill wind blowing through. I eventually figured out that I could use my poncho as a blanket, but that was sticky and stuffy, so I rigged it over the ridgeline from the upwind foot-end to a bit more than half way along, just to give me a little more wind shelter.
This only worked because there was no rain. Had there been rain, it would all have gone inside the hammock and I would have been soaked. But assuming I had two trees the right distance apart and could get the tarp up, the poncho would still have given me a little more wind break than without, and for no weight penalty, because I need to carry it anyway.
Another thing I am considering is a poncho shell. I have a lot of 30D rip-stop polyester at home and it packs tiny and weighs next to nothing. Maybe it'd be worth making a shell that would go round the outside of my poncho and partially or fully cover it. The real challenge is working out how to adjust the ventilation with me already inside it.