My question is -- assuming that you could not build a fire, what can you carry that is light enough (most hunters would not carry a heavy pack) that you could wrap yourself to make it to morning? Would a heavy mylar blanket (not a space blanket but the heavier kind with one red side and one silver reflective side) be enough?
Assuming that you cannot build a fire then a sportsman type heavy duty mylar blanket probably wouldn't do. A dry bag containing a Bivi Jacket (British Army), Bivi Trousers (British Army), Wool socks (Marks and Spenser X-static wool), Goretex Socks (Gore Bike Wear), Primaloft insulated gloves (Trek) and a Fleece lined Gortex hat (British Army) would help though, especially if getting soaked to the skin has occurred. It does mean lugging around about 3 1/2lbs in weight though.
A Dry bag containing thermally insulated clothing)

Even in temperatures just above freezing a Goretex Bivi bag and insulating mat i.e. Thermarest would be welcome as chopping down tree boughs is hard tiring work and the calories expended are better left to keeping you warm during the night. A hot meal and drink is always welcome especially if recovering from a dunking in freezing water.