Is it a 1000? If so, have you tried pulling the access panel that is located where a person's toes would be if they were sitting in the middle? I did this to my Pioneer 1000-5 after reading on Hondasxs.com about it and while I haven't had it in sub 20 degree temps, works pretty well. The panel is located right behind the radiator so air comes across the radiator and blows into the cab. It doesn't blow directly onto the windshield so it doesn't defrost an already frozen windshield quickly but definately puts enough heat into the cab to keep everyone warm and will defrost eventually. Watch out for puddles at speed though cause if the radiator gets wet, it turns to steam and then blows it directly into the cab. Also the slower you go the better, cruising at 30-40mph the radiator cools off pretty quick and then you're just blowing cold air into the cab. When the engine/radiator gets hot enough to kick on the radiator fan, it really blows nice warm air.
It's not a perfect solution but is free and easy to try.