Tom,

My experience tells me it would work. The nights I mentioned, spent in my lofty clothes and without sleeping bag, were quite cold, I cannot say for sure how cold but certainly below 0C since the water in bottles I had inside of tent froze.
I was getting somewhat cold during early morning hours, especially my feet as they were not properly insulated (just some thick wool socks).
If I had a light sleeping bag to crawl into with all my clothes on I would be quite comfortable.
It all makes sense to me, there is much thinner layer of air you have to keep warm when wearing properly sized insulating clothes (puffy jacket and pants, socks, gloves) in comparison to sleeping just in a bag with some midweight clothing. Quilt would not work for me, I move a lot during the night, and I would keep losing warm air as I toss around.
There is one more advantage to this approach I have, I also tried one of those extremely light emergency bags with reflective surface, those weight nothing and work incredibly well, the only problem is condensation inside, so if it gets extremely cold I could put bag inside it, the condensation would wet the bag and not my clothes. Such contraption would let me sleep in even colder environment.