You could go the skyship route. Technically, the sky is made of floor, sorta. You cannot build a wall on the topmost Z level, because the floor that the wall creates would interfere with the above "sky floor". This also means you can build a support at the topmost level, and it will hold up anything under it. Unlike a normal support, however, it will only support the column directly beneath it, thus making it possible to suspend a large brick, but nothing else. This can be circumvented with a support chain, as seen from the side:
SKY
I <-Support with floor underneath
I <-Another support, linking to the above floor, not the sky
---<-Regular fortress starts here!
This way, you can hang your fort from the sky and drop the entire map at will. You'll likely end up with supports on the underside of your fort as the final breaking point, and this means you'll be holding up the entire map by the foundation of your fort at some point.
Do try this on a 2x2. I tried it on either a 3x3 or a 4x4, can't remember, but digging out a whole Z level really messed with the FPS. Atom smash your stone often and try to keep your major population locked away from the main map, as their pathfinding will be killer.
Also, as a side note, doing this is supposed to reveal the tiles that fell, so in theory if you did this, you'd know where every mineral and ore on the map was! Assuming you don't simply drop the entire map into the magma sea, of course...