I've head of wrapping the tubing before exiting the aircraft with either fleece sleeves or aluminum foil (apparently the fleece sleeves worked better). Pre-warmed IV before scene landing - tuck it in with pressure infusers under the blanket as soon as possible on-scene - place in warming device as soon as possible in aircraft. However, it's been many years since any of our staff have dealt with cold weather evac, so there might be better options now. Due to the small diameter of the IV fluid tubing, it is more prone to freezing - even with prewarmed fluids - if you can prep the bag and fluid tubing before landing with insulation it will help.
When all else fails - go to flightweb and find macgyver. I think he's up in Iqualuit or Yellow Knife - and is a great resource for cold weather flying. He might be able to give you some manufacturer names for devices he found handy or - like his name - have some unconventional remedies.