I agree it's a bad idea. The viewing angle will be awful. And indeed, trying to tuck away the cabling will be a nightmare. And even if a fireplace produces almost no convection heat, it will warm up, unless it was insulated to prohibit this exact effect. I cannot imagine this being great for the TV either. It'll a least knock off a few life expectancy years since we all know there's nothing worse for electronics than heat. (well, apart from water maybe )

So I agree with the others: don't do it! Talk them out of it and go looking for alternatives, maybe a beamer with a projection screen dropping from the ceiling, I don't know. To quote Chris Rock on this one: Yeah, you could do it (...), but that don't mean it's to be done! Shit, you can drive a car with your feet if you want to, that don't make it a good fucking idea!