I do not believe Alan actuall meant 'on' the half wall. I am picturing that wall only about 4ft high, although if you could have the wall built out about a foot and then drop to half, that would give you room to mount the speaker and not break the open floor plan.
For 7.1 I would move the 'arrowed rear channel closer to the secctional and then the other one equal distance from that corner toward the L surround position, probably pretty close to where the corner of the island is, although I am sure 6.1 would be more than enough with 3 QS8's.
