Gentlemen, attend. I wish to report a partial success (!)
Instructions:
1. Go to arena mode.
2. In the topleft corner, position a diving bell over the extra-deep bit of magma as follows:
2.a) 3*3 obsidian on z-level (+1).
2.b) 1 empty space, filled with water and dwarves to taste, surrounded by obsidian walls on z-level (+2).
2.c) Same as for (b), sans dwarves, on z-level (+3).
3. Let it drop.
Results:
In tests so far, a near-perfect diving bell is formed, although it causes splashes of obsidian randomly around the place too.
The single-tile floor of the diving bell is completely dry of water, presumably flash-evaporated.
The single-tile floor of the diving bell is completely sealed off from the magma, and a couple of z-levels down below the magma.
The single-tile floor of the diving bell is also, sadly, completely free of dwarves. They get thrown up in the air and shaken all over the place in the cave-in process. I predict that a deeper tube, if allowed by the arena, would prevent this.
Observation: adding a third ring wall, the next z-level up, causes the water column to cavein first and smash through the magma, turning into a pillar which holds up the obsidian. Which is no good. It may depend on the update order of the terrain tiles and water tiles.
Next step: I've heard it's possible to alter the arena. If we can make it a huge cube of magma, do so. This will let us drop much larger, more complex diving bells into it, and hopefully let us design one where the dwarves stay on the inside.