I'd look for a Thinsulate lined work boot or hunting boot instead of a snowmobile or Sorel type boot. I think that the orthotic would work the best in a winter boot that most closely resembles a normal summer boot. I don't know how warm a boot like that is, but I'm sure it would have the best support and be most comfortable with orthotics. I know that I saw the late, great, Rick Ward wearing regular leather workboots on the hillclimb circuit, so don't let anybody tell you that sleds can't be ridden without snowmobile boots. I doubt that whoever tells you that is a better rider than Rick Ward.
1990 Indy 400 (Slightly modded with welded tunnel cracks and various used Indy parts from models throughout the 90's)
2006 Stihl Chainsaw
1980's Vintage Lawnboy Lawnmower
2005 Husqvarna Weed Whacker (It sucks to rewind the string, but that 31cc motor sure is torquey!)
2000 Polaris XCSP Edge 600 (1.5" Track, 22x40 gearing, Aaen exhaust, Student Driver pivot riser package)
2008 RMK 600 144"