I've seen it done in clothing - vests and such - mixing leather with quilted panels or tapestry, but not in a quilt. Would be a beautiful effect though. If you've not worked with leather before, there are special leather machine needles you can get, that punch through it more easily than most. Ditto for hand needles, get the triangular leather needles, your hands will thank you! Can't wait to see the ideas come out!