I personally don't think it matters where the framebuffer resides, as long as it's writeable from the kernel (which it is of course). All you'd do is alter the position to which you write.

As lcuk says, there is a bandwidth problem, so rendering full screen is limited (YUV is quicker than RGB, etc.), but that's simply a platform limitation, it shouldn't stop the hardware acceleration from doing its thing, accelerating 2- and 3D transforms and the like.