Not a bad idea really. I agree it sounds a bit dangerous. I think I would modify the idea by using a metal can with a metal lid and a few small holes drilled in the sides of the can for air. That way the coals would keep you warm and not ever flare up to burn something important.
After all ,lets not forget back in the days of "Laura Ingalls Wilder" they used to heat up rocks in the stove and put them in a special covered pan that could be put under the blankets at night to keep them warm in their "Little House On The Prairie". This idea is just a version of that.
I guess I've always stood around the campfire in cold weather and did what we call the "Rotisserie" . But, if you want to set by the campfire then this idea has merit.
