Just turn a reciever on the side, no moving parts and as long as you do not block the vents it will work just fine. The problem would be not seeing the display and remote but you could just stick a mirror there it would reflect the IR of the remote and show the display (backwords and side ways).
Just check the reciever after 2 hours of use and see how the heat is displacing since heat rises and you have changed how the heat is disapated, you may have to add a little 2" or 3" PC fan on top to compinsat it sitting on the side. Also make sure it is stable maybe some rubber feet, they would raise it 1" or so off the ground and help air flow too.
Other then that... No one makes a reciever that way, maybe because then you would see the mess of cables? Maybe heat?? There is probably more then one reason.
Does any company make a video/audio receiver that does not stick out 90 degrees from the wall. Maybe two or three inches in depth, 12" or 14" wide, and maybe 12" or 14" high. If anyone can help it would greatly be appreciated.