Earlier quoted context omitted.
most TVs are then still using almost full power, some still aren't "black" when a black screen is up (it provides light at night, enough to see with in my case!), and still puts off heat. It's a hack that might work for something personal, but I wouldn't be comfortable selling a product that did this.
Fair points. I'm really kind of tied to the idea that someone is going to do this as a set-top box format. What you mention is a major issue though. Maybe it can use the video output only if the TV is already on, and just failsafe to audio in those cases, like a regular echo.
But I would be interested in a device that could pull it off. Doubly so if they were cheap enough to get one for each TV.