I used the Nest as an overquilt when I was in the Rockies outside of Durango this summer. Spent a week in a hammock, with a pad underneath and the Nest over me, down to around 30F. I was warm and comfortable.
As long as the quilt allows you to form a foot pocket it can be used as an overquilt. Personally, I'm not sure I'd want to use a flat quilt over me if I couldn't create a foot pocket, but it would probably work also.