I think it might be a little more difficult than that -- finding a driver that works well with a shallow "cabinent" can be difficult I'd imagine. Madisound has this
one as well as some
coaxial drivers that'd work. I just think most drivers would need too much back clearance to work well in wall. I don't know if there are any other issues that would need to be addressed.
As for a crossover, I'd imagine removing the baffle step compensation (BSC) would be the only issue.. I think.
I know I'm not being too helpful.. I'm just giving whatever info I do have.
JCD