Thank you for the responses. It might be possible to install in-wall speakers but that would look bad because the rear-right would probably go in the middle of the wall. I hadn't considered a third speaker and perhaps a single stereo speaker such as NS-ICS600 would work in that central-corner...
In our living room we have the 60" TV flanked by two MK-S85 speakers (left/right) and a MK-S85C below the TV (center). The TV is in the corner of the room and there is no where else we want to place it. I purchased a pair of Yamaha NS-IC800 ceiling speakers to work as the back surround...