The lazy susan idea would probably be easier. The method you mentioned would probably have to be designed yourself. It would not be impossible, but it could be a lot of work.
I wouldn't install cabinets that stick out past the mantle either. I think your best option is to try and find devices that might fit on the mantle. There are quite a few small dvd/bluray players that are small enough to fit. The xbox might be a harder issue. Turned sideways in the standing...