Using the heat from a larger TLUD is hard because of the height.
Lowering it into the ground seems like a good solution.
An inground firepit can easily be used to make charcoal, but only as with the flame cap method, and that needs tending.
To make it an Up Draft kiln there needs to be air injected into the bottom of the pit.
So how to do this?
I m not sure it can be passive or natural draft, but lets suppose it can be fed with a battery powered air pump, like a
mattress inflator.
In a permanent scenario, I see the top half a barrel buried flush with grade.
The
wood goes in a metal basket we make from hardware cloth, and that sits on top of 3 bricks.
The air is delivered into the bottom of the barrel by way of a metal pipe that is run down the outside of the barrel and over to the center of the pit.
Pipe fitting, etc are used to fit your pump to the pipe run.
We might need a diffuser plate to distribute the air evenly.
To use the pit, light the top of the wood, and turn on the pump.
When the wood charge has turned to charcoal, turn of the pump and cover the pit with the lid from the drum.
Plug the pipe.
Come back later when everything has cooled, and hoist out your basket full of charcoal.
To use the heat, we can use the bottom of the 55 gallon drum.
We invert it, cut hole for and attach a flue pipe, plop the whole thing over the pit kiln.
Cook on or over the resulting hot surface.
If the bottom of the flue pipe extends down into the barrel, we might be able to use turn the barrel into something of a bell.
Now this is all well and good, if it works. but what if we just want a pit full of charcoal to plant into?
In that scenario we don't need a basket, and we probably don't want pipe permanently buried in the pit.
We can chucks of wood at the bottom of the pit, and build a "floor" of planks over that, if necessary.
I think our pipe just goes to the middle of the pit and 90 degrees down to the bottom.
That is probably a better design for the permanent pit as well.
The rest runs as described above , though one could do a second burn on top of results of the first, in order to further fill up the pit.
This would need a shorter downward leg on your pipe run, but I think I have an idea for that.
Instead of an "L" shape made out of pipe, make the short leg out of corrugate stainless steel gas line.
That will let it be durable and flexible
enough to be set at various depths .
Totally other idea for automatically dousing these charcoal burns .
Run a metal pipe from your tank of dousing liquid to the bottom of your burn pit.
Plug the end that will be in the fire with wax.
When the heat gets down the the plug, the liquid pours down the pipe, and fills the hole.
This is better for pits you wont be using again.
You could maybe do the same thing with a glass bottle, especially if you didn't mind it staying down there.
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