How about a very shallow mesh cone located in just the upper part of the chamber? Ideally the mist would need to be inside/under the cone. The roots will spread out and cover the top surface of the cone mesh and also hopefully grow down "en mass" to the chamber floor.if you made some sort of cone from the bottom of the net pot to the floor the roots would follow it down but you would have a void inside the cone.
The perfect outcome would be a complete column of suspended roots covering the same area as the diameter of the cone base. There should be plenty of open spaces within the distributed rootmass for the mist to penetrate.
In the early stages the mist will be able to fire through the cone mesh..at least until the roots have completely covered it.
1/4" x 1/4" stainless steel mesh would be a good material for the cone.
At the end of the grow just cut the bulk of the roots away and throw the cone into an incinerator. The roots burn to ash and the mesh is sterilized by fire
Anything on the floor of the chamber is likely to get a surface misting and the rest mostly runoff and perhaps any larger droplets that fall rapidly. If the rootmat is fairly open it will penetrate but that wouldn`t be for very long with a tree