BO...this is so awesome.
This thread is gonna be a classic when it's done.
I've also always had a dream of building a permanent masonry pit at my place. I never thought of doing a direct, coal fired pit like the ones you mention. I really like the idea of having a versatile pit that can also be indirect.
So, for the firebox, are you thinking out of steel? Couldn't you make a firebox out of masonry as well or is that what you're thinking? I wonder how "airtight" the big top doors should need to be if used as an offset, strictly for draw I mean?
You gots masonry skillz? I agree that the foundation is UBER important to any masonry structure. The weight of any masonry structure is heavy. And frost/heave is the arch enemy of any structure, but especially masonry ones since any movement at all will cause cracking. So either you need to get below your local frost line or you need to make the base/foundation sturdy enough to move as one big piece in the case of frost/heavy and not "flex".
This is gonna be good. Watching intently.....