I now have two of these Beurer HK 72 Akku belts. It has wires like in an electric blanket. I bought it for exactly the same reason, as I was clutching a hot water bottle to my lower back.
In the interest of science I have just tried it on my chest. It is a bit tricky with my bust, but I think it would work!
I bought it for bowel problems, it seems to help with cramps. I'm having a bad patch at the moment and I am wearing it pretty much 24/7.
The battery pack is in a little pocket on the right hand side, and so I can only sleep on my left at night, and it sometimes digs in a bit when I bend over. But generally it is very comfortable.
The battery lasts about four hours. When I started using it more frequently I ordered a spare battery from the Beurer customer services, so I can have one in use and one charging. And then more recently I bought a second belt.