No need for boot heaters. We use warmed up rocks placed near a fire or on a stove. Wrap in foil if they aren't clean. Don't get them to hot.
Drop them in the boots or even fill the entire boot with them before going to bed. We have even warmed our rocks up in the morning and had fresh warm boots before heading out for the day.
I imagine these fans and a couple warm rocks would really dry a boot out.
Interesting idea. Anyone have a weight on their setup (minus power pack)? I wonder if you could pair these with those zippo handwarmers(2.5oz) if you were in a really wet backcountry hunt (some AK hunts come to mind)