I've recently had the chance to use my new winter hammock rig in some solid sub-freezing weather.
My current rig is an ENO Double Nest Hammock with a closed cell foam pad and a DIY underquilt my wife, Sleeps With Skunks, made. It is from Kickass Quilts Hennessy Hammock model, made from 5 oz Primaloft Sport insulation. Above, I've been using Speer's Winter Tarp.
On my recent trip on the Foothills Trail in South Carolina, I used this system down to 18 F in wind gusts of 30-40 MPH.
I rigged the tarp at its normal height, with the Speer Tarp low to the ground.
It made it a bit tight to get into the hammock, but it offered excellent protection from the cutting wind.
Once inside, I was so warm with my home-made down quilt that I actually had to shed layers after the first couple of hours. But I definitely hammocked happy.