I've done it in a stitched through summer quilt. I cut a 22" slit at the center of the mid-length quilt line where the down would be thin anyway. Then I stitched-in a 2" high down filled collar that closes with omni-tape. The collar sits in the valley where the normal quilt line would be and is actually warmer than the other stitched though quilt lines.
The quilt uses omni tape to attach to a tapered 'casket-cut' shaped ridgerest pad with mating velcro tape stuck to its bottom. The omni tape sticks to itself, so it forms into a warm, but ill-fitting vest or serape. If I had to do it again I'd have used 1" finished net baffles as the sewn through design compresses the down so it doesn't loft fully.
In a similar synthetic quilt I used a round hole and a very tall 4" collar that drawstring closes to fill the 8 inch hole. This was a single layer of 1" Polarguard and the joint between the body and collar has zero loft. No problem for summer temps at low altitude. But the straight slit hole is easier than the round fitted neckhole, and since a serape looks like crap anyway, the primary function of quilt is less disrupted by a slit and short collar than having a drawstring-closed 'port' on top of a quilt.
Since I bring wearable backup clothing the vest serape feature turned out to be redundant on a cold weather trip (40 F) where the insulated clothing made the light quilt workable, but then not needed for camp serape wear.
I wound up throwing the quilt over 3 people's legs during breakfast, but that could have been done with any sleeping bag. With a communal breakfast under my tarp, the quilt was just closer and handier to ward of a chill.