I figured a snow bank and the ambient air temperature would cool the work quickly with out the need to bring out the wort chiller.
I have the same setup as you...the Bayou SP10 and 40 Quart kettle. I usually try to get right to 7 gallons preboil, but then add the no foam stuff to avoid boilovers. I'm in Southern NH so I feel your pain. Your biggest problem will be the wind. Even with the little heat shield around the kettle, the wind blows the flame off the kettle and makes it hard to keep a consistant boil.
Placing your kettle in the snowbank may chill it, but if nothing is transferring the heat, the snow bank may actual insulate the kettle. I would use the wort chiller, and put your kettle in the snow. Then once below 100 ( probably ok below 140 but I hate to take chances ) I use a sanitized spoon to stir the wort gently to bring the heat down. There are some who have posted here that say if you lift the chiller up and down in the kettle, it will help chill it and aerate it too. ( again..once below 100 )