If you are considering the QS series for your surrounds, you'll find they are very forgiving when it comes to placement. The following is from the Axiom FAQ page.
"Where should I place my surround speakers?
Ideally, surround speakers should be either wall-mounted or stand-mounted to either side of the couch or primary listening area, from one to six feet higher than seated listeners' ears, and slightly forward or rearwards of the listening area if you wish. Axiom's Quadpolar surrounds are especially versatile and can yield excellent performance with stand or wall-mounting to the rear as well as at side locations. (see our dedicated stands)"
So, while mounting surrounds to the side of the listening location is optimal, mounting them on the rear wall will work just fine. Just try to mount them at least several feet on each side of the listening area. You don't want them on the wall behind you, directly or nearly over your head. From your drawing, I'd guess you could get them spread out enough.
