I've never used a quilt so can't quite picture what you are getting at, but one thing you could try is simply draping a jacket over your arm if it's outside the quilt. I've tossed my down jacket over my bag when it got colder than the rating (big down parka-the one in my picture) and that works pretty well. If you want a hood, a couple of companies make separate hoods. I think Ray Jardine has one and here is another one-
http://www2.smumn.edu/deptpages/~finnovations/Hood/hood.html