The animation Peter linked to is why I mentioned adding the raised "obstacles" to the face plate. We know that the sound waves experience refraction as they pass through the opening, and that they tend to hug the surface.
What I was suggesting was to design a faceplate that uses intentional refraction (and reflection as well) to correct the "natural" refraction. Place an obstacle on the exit side wall on each side of the opening. Design that obstacle in such a way that it reflects enough of the sound back to the "proper" direction to fill in a good bit of what is lost, and also to refract (or diffuse) the remaining sound energy away in a "diluted" fashion.
Another possibility is to change the shape of the opening. Beveled in, maybe? Before they get to the exit, relfect inwards the sounds that will be bent outwards as they exit the speaker. Come up with a way to produce the sound inside of the speaker in such a way that it needs the effects of refraction from the wall to sound right. Kinda like "folding" up sound, and letting refraction "unfold" it for you.
How about putting a disk in the center of the speaker? The edges of it would refract the sound waves inward towards the center. Maybe the outward refraction of the wall plate would undo the effect and it would end up sounding right.
Just pondering...
