Non-relativistically speaking (danger will robinson!), the photons emitted by the radiator have a huge gravitational potential associated with them. You're free

Non-relativistically speaking (danger will robinson!), the photons emitted by the radiator have a huge gravitational potential associated with them. You're free, I believe, to extract that energy through a heat engine if you like, but I'm not sure the how feasible or efficient it would be. I'm totally ignorant on the GR front; where's RobotRollCall?