The Sea to Summit Traveler TrI 50 sleeping bag might work - 1 pound, 2 ounces, hoodless, slight mummy shape. Not sure how strong your heart is - but I think $220 in a regular length might be less than a defibrillator.

Have you considered clothing rather than a liner? I've experimented with that over the last 2 yeears, and been very pleased with the results - it let me take my 45 degree WM quilt down to 25 degrees (might have gone a few degrees colder if I had used a warmer pad.) The setup I use is from Montbell: a hooded Superior down jacket, Superior down pants, and down booties. (I also use their down balaclava, but that's only because my quilt doesn't have a hood, and I wanted two layers of down around my head; it also seals around my face.) The bonus is that you can wear the down clothing while sitting around in camp, then wear one piece or all pieces (depending on temperature) inside the sleeping bag. It's a bit more costly than a liner bag, but I think the whole setup was right at $400, which was reasonable for the extra function it gives. (You get rewards points if you sign up for the no-cost program; the jacket and pants were enough points to buy the booties.)