Handwarmers
I believe that heating gloves, hats, socks etc to 65 degrees C or 149 degrees F might be a problem - potential for burning skin? How much heat would be transmitted through the covering?
In addition, since the device would retain this heat for a long time and over a wide swath of fabric, a burn or discomfort factor could occur. Putting an uncontrolled heat source in gloves, hats, socks etc. might be a problem. If you had some sort of variable heat emitting control that would be helpful.
Finally, one of the advantages of the small chemical handwarmers on the market today is that you can use them for short time to warm up, put away and then use later on and they are portable (change hands, pockets, hand or feet etc. and/or person to person) this would be difficult with some clothing items.
Lastly, I am sure not ll deer camps have microwave ovens to activate the chemical reaction.