For treatment to have a strong effect for low frequencies up to a few hundred Hz it needs to be pressure based, not velocity based absorption. What you suggest is also ineffective because it can be accomplished with some amount of porous filler and air gaps, plus some of what is going on in the transition region is re-reradiation from coupling between the walls and structural supports. They all flex with playback, and it's not easy to control those resonances.
Creating an effective cloud also needs a mix of a diffusion and absorption. A simple way to do that would be to use a space coupler (see below) above which you have a layer of porous material. But the key is that you have to calculate how absorption much you need, so that it's kept even. Sabins per frequency. Not that you have to be too accurate, but still. Unevenness is a greater problem and easier to accidentally create than than overdeadening.
Your suggestion is pretty good reminder, though, that absorption doesn't need to be symmetrically placed around the room. All it does is dampen a certain amount of energy, so you can put more in one spot than another, although you still have to account for how that will bias radiation and affect envelopment and imaging. I would imagine
@ripmixburn's space to have some flutter echo that could be dampened by putting something on the walls. But if you have pure absorption for the ceiling and add stuff to walls it probably won't sound great, especially if the speakers don't have the most even directivity.