I wonder if Jeff's design doesn't solve this problem.
He has a hopper on top, connected to the reaction chamber by a section of pipe (maybe 8" in diameter?)
The pipe protrudes down into the reaction chamber a little bit (2 or 3 inches?)
So, the fresh charcoal would always be falling into the middle.
And he picks up his gas in the void between the sidewall and the pipe at the top of the reaction chamber.