From reading here I think most quilt users cover themselves up to the neck with the quilt then wear some type warm hat for the the head.
Would it not also work in reverse? That is, have the quilt cover from, say, the knees upward and then wear some type of small, narrow warm bag for the knees down. Could even use one's backpack. The upper part of the quilt could then be shaped like a conventional sleeping bag where it fits around the head and neck (with an opening for the face).
I'm not a quilt user but I use me sleeping bag like a quilt. I'm reluctant to give up the top part of my sleeping bag because it is so darn warm. My feet, however, are usually too hot and spend most of the night outside of the bag anyway.