Won't the surrounding upright bridges prevent the formed obsidian from attaching to walls?
And how much water does it take to cool magma to form obsidian? This is potentially awesome. Cave-ins just make a rock when they fall and break, right? No natural wall left behind?
Natural wall cave-ins leave behind natural wall.
Construction cave-ins leave behind rock.
My suspicion, though I can't confirm it, is that the obsidian could/would attach to the same surface supplying support for the retracting bridge.
You COULD, in theory, make a device that reloads with dorf-intervention:
Build constructed floors on the level under the bridge, and place a support in the middle of those constructed floors. If the water always pours directly onto the obsidian block, it may be supported by the support underneath.
Then, you mine/channel out the edges of the plug.
Then, a lever retracts the bridge, and another removes the support.
Rebuilding requires replacing the floors, the support, and remining the plug.
Questions:
Does a support provide support through a bridge that is deployed?
If it does, and you form solid obsidian on said support, what happens to the bridge when you channel the obsidian floor that is resting on it?
If the answer is "Nothing", then this can work.