I was kicking this around, looking at using light liners in conjunction with an AMK Thermolite bivy for a quick and dirty summer bag. I remember being surprised that a simple fleece bag could be so heavy, but they are. If you think about it, a basic 200W fleece pullover is usually in the 16 ounce and higher range and a sleeping bag is going to eat up some yardage. No free lunch!
Sea to Summit makes a Reactor Extreme Thermolite sleeping bag liner that is supposed to add 25F to your sleeping bag and weighs 14oz and runs $60-$65. Trouble is, my 40F Mont Bell down bag is only 16oz, but that isn't cheap or MYOG-- I did get it used :)
I've wondered about tearing down a car camping style sleeping bag for the batts. Construction-wise, a quilt seems to be the way to go.