Just experimented with a small embark: only 5 z-levels above ground, and invasions turned off. Mixed results: it looked promising, but I've yet to find a method that will actually support hanging forts.
I have not yet managed to get to magma (aquifer in the way) but I've been able to build wooden constructions up at the top level. I started with a stair stack (2x2 square of constructed up-down stairs) at all levels except the top. At the top Z-level, you can't build an up-down stair... but you can build a down stair. As for the rest:
You can't build walls on the top level, that is sure.
can you build floors?
Yes. Floors can extend as far as you like away from your stair stack, and provide support to each other.
I built a long chain of wooden floors, then removed the next-to-last in the chain, and the last one caved in. The Top-of-the-World ceiling does
NOT provide support to floors.
more importantly, can you build bridges?
Yes. 1x3 retracting bridges built just fine. 1x3 drawbridges could also be designated, though with my limited stone I didn't have enough mechanisms to raise the drawbridge.
the question is, can you get anything at all that can support further constructions to hang off them and create dwarven floating castles?
You can build supports on top of a constructed floor. The support attaches to the Top-of-the-World ceiling, and supports the floor it's on.
HOWEVER, this does NOT extend to the Z-level below.
I built a long line of floor tiles on the top Z-level, built a support on the last floor tile, then removed the next-to-last floor tile. Unlike my previous experiment, this time the floor+support combo did NOT fall. There was nothing touching it orthogonally or diagonally, and nothing underneath it -- the only possible place it could be getting support from was the "sky ceiling". But when I built a bunch of walls on the Z-level below (hanging underneath the floor tile), they were not supported by the floor tile above.
The only thing I haven't tried yet is building a wall at Z-1 from the top, then a support at Z-0 on top of the wall. I'll report back once I've tried that.
Edit: That didn't work either. The wall
directly under the support stayed in place, but the rest of the walls and floors touching it fell once I removed the last bit of floor connecting them to the stair tower. This proves to my satisfaction that:
1) The "sky ceiling" can provide support for a tile DIRECTLY connected to it (via a support or a wall), and
2) this support does NOT extend to other tiles, unlike "normal" support.
About the only sky construction possible, unless someone else finds something I missed (quite possible since I'm pretty new to DF), would be a "surveillance platform": a forest of supports holding up a floor. But no stockpiles would be possible, which means no booze, which means no dwarves -- not for long, anyway.