in the 1960's and 70's a lot of quilts were made of double knit. Colors stay bright and they wear like iron. I would not use denim for backing as it would make them heavier and for me harder to work with.
Probably good use for charity quilts. Check out a group that makes "Ugly Quilts" I guess it is still around. That program made sleeping bags for the homeless. They did have a web site.