It is true that the instructions say not to sleep with the Thermacare on you. Most electrical hot pads have the same warning. For some people, particularly those with compromised circulation, the heat directly on the skin can, indeed, result in a burn.
As a warming device in your sleeping bag, the product does not need to be directly on your skin. It could be over a layer or layers of clothes. It could be wrapped in a poly jacket. It could be shoved down at the bottom of your bag. It still pumps out heat inside the larger insulating envelope of your bag.
Now, before doing so, you probably should use one, on your skin, during the day to see how you, personally, respond to the output. I, for one, have no issues wearing one directly on my skin, and have done so many times when my back has been acting up. Once assured of daytime use, you can try it inside the bag, but off your skin. I am now confident of my personal response, and wear it right in my back.
Two things to consider further. One, I normally sleep on my sides, not my back, so I don’t have the product sandwiched with my body weight between me and my bag/pad. Second, I’m usually at 8000 feet or higher elevation, so the reaction rate may be lower given the reduced atmospheric pressure.