It works! I made 7 pounds of biochar from 28 pounds of firewood.
The top down burn is the easiest campfire you will ever make. Crisscross layers of wood and light the top. The top layer will burn. The falling embers will eventually light the second layer. Slowly, the fire will make its way down to the bottom layer. You never have to tend the fire. And it burn cleaner than other campfire methods.
When the fire gets low, quench the hot embers to make your biochar.
Here is a video:
https://youtu.be/nFKSaeudqCQ
I hope others try with different fuels and report their successes and failures.