Look on ebay, etc., there are some drop in generic 12 volt flexible panels that are intended to warm SNOWMOBILE seats. Arctic Cat as an example sells a touring sled with a heated seat, it's flexible enough, no worries.
The ones I saw on ebay and such were about $50 bucks.
I would want to rig it to a handlebar switch and perhaps cycle off my grip warmers while warming my ass, and then turn off the seat heater for a bit while warming the hands to minimize the draw of power.
Just some random thoughts of a complete mad-man.
Here is another way to go:
Ps: Here is my favorite cheap ten dollar handlebar generic switch that can be used for almost anything. Small enough to have two of them side by side, one for driving lights, one turns off heated items, ass toaster, etc.