I wonder if a DIY Insultex UQ might work. Fashion an end-gathered hammock-like structure the length you might need and string it up under your hammock body with shock cord?
I'm new to this but it seems that layering might work with hammocks like it does with clothes. If you go to the closet you see many options. My mom used to tell me, "Always bring a sweater." So, I wonder what might be the functional equivalent to a sweater for a hammock.
Now, I've seen snow in June in Estes Park, and sleet in the Pecos Wilderness and Slumgullion Pass in the summer. But it was short-lived a camp fire made me feel better.
My biggest consideration at this point is financial so have studied closely many options as I transition to UL camping gear, of which a hammock has become a major part. I just happen to have an extra Marmot bag to convert to an UQ, along with a LL Bean discount down parka to make into a 3/4 UQ. But $39 for an IX hammock sweater might be in my future ... if only as an experiment. Those are like coats. For summer you need a sweater at elevation.