I love this idea! You can get the tape from hardware store (i.e. Lowes or Home Depot). It's pipe electrical heater tape. It plugs into a power source. You only need to be concerned with core temperature around the heart. You don't need to worry about back or extremities. Your heart will carry the heat throughout your body. You don't need it too hot either. I think affecting your core temp by 7 degrees (F) will be noticeable to the user. Don't want to go too high and be hyperthermic. Some sort of thermistor regulator would be good. You could power it with a lawn tractor battery carried in a fanny pack. If you could get a hold of some flexible solar cells (i.e. nanosolar) or a small Harbor Freight solar cell you could recharge this in the field by standing in the sun for awhile. I've seen a similar idea from Sharper Image with a unit around the neck (uses the Carotid arteries for thermal conveyance).

Keep up the good work! I'm giving you a skull...
SOTB (Chris)