I remember at the Exhibition last year I saw these little gel packs that are stiff and when you break or bend it, it heats up ? Anyone know what I'm referring to? At that moment I thought about how it would make a good solution for thermal wear inserts when you're riding... and then the thought passed as I got distracted by the next exhibit... lol.
Something like this....
http://heatfactory.com/Activities/Snow_Sports/snow_sports.html