I've been thinking more about going the quilt route. I'm in a hammock, which means that if I want to stay warm in cool weather, I need some insulation under the hammock, so the insulation in the bottom half of my sleeping bag is: a) redundant, and b) squished under me, so not all that insulating anyway. I have a summer-weight mummy bag, and slightly colder-rated rectangular bag. Like the OP, I don't like mummy bags, so my last trip involved my rectangular bag, which is bulky and warmer than what I need for summer use. With a more expensive, more compressible bag, I could get my pack size down and still keep the rectangular bag but I have found that rectangular, light, warm, compressible bags are the minority. And you still don't really use that under-insulation. So a bag with a pad pocket or a quilt seems to be the way to go. That's probably my next gear purchase, although I'm not in a hurry. Quilts are a little pricey, and I'm not short on options to stay warm, just on space.

The other item I have that's been really helpful is a CoolMax Travel Sheet. I've used it to supplement my summer-weight bag in cooler temps, and I've used it as my only insulation in really warm weather. The fabric is more comfortable then my sleeping bag fabric or the bottom of my hammock, and sometimes that little bit of insulation is all I need on a warm night.

So I'm all for a quilt if you can afford it, but for now I'm using my CoolMax liner and supplementing it with my summer-weight mummy bag, unzipped. It's not the perfect solution, but it does the job.