I would recommend that you avoid traditional big box speakers that are "in-wall" models as most of them are sub par, at least in my experience. I have heard in-wall speakers from the likes of Def Tech, Klipsch, B&W, and several others. To me, they all sounded pretty bad.
You could build some of the Seos-15's or Seos-12's, or Seos-Tux 1099, or what ever Seos speakers you want. Then recess them into a baffle wall, or build then into the sides and rear walls, in addition to the front walls. I believe that most of the Seos designs will work well recessed in a wall, as will the 4Pi too. If you go with the Seos based speakers built flush into the wall, you will likely loose the effects of baffle step, and gain in the basic acoustical properties of the speakers. The downsides to doing your front, Left, Center, and Right mains sitting flush in a baffle wall would be that you loose the ability to toe them in, although that may prove to be a fairly insignificant factor as there are so many other advantages to flush mounting the speakers in a baffle wall, as well as the inherent benefits of controlled direct ivory speakers in general!