i think firebricks, or even baskets will work just as well.
the effect of controlling the airflow that specifically isn't really worth the effort and costs involved.
air molecules diffuse very rapidly. person next to you farts, you smell it.
I'm wondering if directing all in coming air to the coals will make it more efficient. Say I have an ambient temp of 60F and am using a basket, 60F air is going to need to be mixed with heated air from the coals. If all air can be directed to the coals, cannot a smaller quantity of air be heated to the same temp? It seems that there is an inherent inefficiency to having most of the air bypass the banked or basketed coals. Air that will also need to be heated to maintain cooking temp.
Baskets and bricks will not do this. Foil might, at least somewhat.
I don't think this is how thermodynamics works.
I am no expert, but here's how I see it.
There are two things happening in the kettle with banked coals.
The first thing is a simple chemical reaction. The charcoal is oxidized, which pulls oxygen from the surrounding air and outputs heat and carbon dioxide. If there is not enough oxygen, the burn is incomplete - which produces carbon monoxide and creosote.
The 2nd, is the heat radiation. Heat is going to radiate in ALL directions from the source. Not just up. The metal or firebricks we often place next to the coals are insulators. They're going to slow down the most direct heat, but not stop it completely.
The air on the other side of those bricks or that steel is GOING to get heated no matter what. Luckily, air doesn't take much to get heated. It will absorb some energy and quickly rise.
With everything being sealed off and only the coals having direct access to fresh O2, I would think that the dead space under the meat would not have any airflow. It'd be stale heat and stale smoke.
Nothing really changes in the chemical reaction with the charcoal. It's getting its O2 just like it would have without the dead space (or even the fire bricks)
The only thing more efficient is your vent, but your coals are going to need the same amount of air.
Heck, that dead space could mean that your food is getting more stale smoke than fresh. It could be altering the cooker for the worse.