I tried a bag liner one time and, since I'm a lively side sleeper, it was an exercise in entanglement. I've been wondering how it would work with a quilt. In theory, I wouldn't get nearly as wrapped up. A 20F quilt with a liner should get me through any hunting season I'm likely to be out in.
Read the literature. I can't remember the brand but a bag I looked at a few years ago had an interesting note about temperature rating. Most of us think of a 20F bag as being more or less comfortable at 20F. This one said that they rated it as one that would keep you alive at 20F. Not comfortable, just alive. It was probably a 40F bag.
This would be my experience, my North Face 0F rated bag was not comfortable in single digit temps. Not at all.
You folks who are trying the quilt option, I'll sure be looking for your posts on how they work out. I, too, am not much for laying out flat and sleeping in a tight bag.
For truck camping I'm actually thinking I'm going to sheepskin rugs and wool blankets.
Geno