This sound so awesome and so insane to design I'm not even trying
I've done it, and it's very practical. They're not completely separate - there's a staircase from the main fort just below the surface to the iron mines between the caverns and the foundries on the magma seas. But I have happy dwarves in the foundries, and happy dwarves at the surface, and happy miners in the mines. Burrows being used for what they were originally intended......
I'd be really interested in hearing any details about your setup!
A decentralized fort system is something I've always flirted with but could never quite get to work properly. When I don't use burrows, the dwarves are terrible at choosing which dining hall / booze stockpile / clothing pile / stone to put in stockpile etc. to use, only picking the closest one about half time; otherwise they make the 100 z-level trek to the opposite site of the giant staircase, wasting tons of time and defeating the purpose of the separate forts.
When I
do use burrows, it's arguably worse. Since burrows just restrict which jobs a dwarf can actually
do, but not which ones they attempt, I get flooded by job cancellation spam from dwarves frozen stock-still holding half-empty beer barrels, because they've chosen the drink stockpile at the magma sea to deposit their drink (which is outside their burrow), instead of the stockpile 1 tile away they just picked it up from. Similar situation for putting other items back in stockpiles (worn clothes, used thread from hospital, seeds, etc.).
Is there a way to make a pathway of some kind that minecarts can traverse, but is literally inaccessible to dwarves? I feel this would solve a lot of my woes (albeit with a bit more of a complicated system), since the dwarves wouldn't attempt to put an item in a stockpile that they physically can't path to (I hope?).
EDIT: I imagine using roller-coaster style jumps via ramps that sent the cart flying in the air over a chasm could fulfill my requirement of allowing cart movement, but blocking dwarves. Are minecart ramp-jumping physics reliable enough to make this a consistent system?
EDIT II: The legend of Curly's Gold:I think I'm over-thinking this. I could just make the magma-forge fort a sealed cube, with a hole in the ceiling, and a hole in the lowermost floor. A minecart automatic quantum-dump can dump raw materials and food into the hole in the ceiling, and a similar setup can dump the finished metal goods out the hole in the floor (which could then be scooped up into yet another minecart to bring it all back to the surface). This would keep dwarves totally sealed inside, but allow materials to automatically and easily flow in and out of the magma work area.
I'm still in 34.07 so... would this work?