For winter camping, a wool blanket is probably insufficient as a top quilt, unlesss it is very thick (which means very heavy!)
As far as the warmth from the TQ is concerned, treat it exactly as you would while tent camping. Assuming you had an appropriate pad underneath you, would a wool blanket be sufficient on top?
For me when camping in the 0 degree Celcius range, I want at least 2-3 inches of loft on top. However, depending on how weight and weather concious you are there are plenty of TQ material choice available. In the worst case scenario you could use a cotton or silk duvet cover with a single/childrens size down comforter inside. Obviously that won't hold up like nylon/poly will and might be heavier, but it is an option.
However, I agree that it shouldn't be a problem to get one of the cottage guys to make you something custom, and I further agree that you will save a lot of $ in shipping and duties by filling the down locally.