It seems to me that a low-voltage pad heater would fill the bill. The
following link is for a company that makes pad heaters much like the
Tanis, but designed to operate on 12 Volts:

http://www.padheaters.com/sizing.html

A resistance heating element doesn't care if it is run on AC or DC, so
a battery would do the job. You could charge the battery with a solar
panel - and rig up some sort of means of turning the heat on remotely
if you want to get fancy. I have heard of people using modified cell
phones for such things. A pager modified to provide a contact closure
would work as well. You'd need to do some experimentation to work out
pad wattage, battery capacity and solar panel size - but I think it is
a practical solution.

David Johnson