As a Canadian it is often cold out when I cook using my Dutch Ovens outside. Additionally, the ground is often frozen so I don't have the option of digging a hole to cook in. That is the price of living in the North!
My personal technique to combat the cold is to ensure that I have a good wind break and then I add a couple of extra coals to make up for the temperature difference. If I am using briquettes, I add 2 extra briquettes (or more) to both the top and bottom, if I am using hardwood coals, I add a similar amount of red hot coals to both the top and bottom. One other thing that I do is to change out the briquettes/coals more frequently so as to ensure their quality of heat. It requires more coals, but ensures an even and consistent heat. I have used my many Dutch Ovens frequently in the -20 to -30 F range and have not had any difficulty in getting similar results to those that I do when cooking in summer time temperatures.
I hope it works out for you, and thanks for planting the idea to do some Dutch Oven cooking - might just have to get after it this coming weekend.
Cheers,