I tested it out and weird shit results i have to say.
Well, first of all, the bridge sadly doesn't support the block of obsidian, which isn't that bad.
Secondly, the block of obsidian doesn't shatter, which again isn't that bad.
Now for the bad part, i tested it multiple times and only once i managed to create an obsidian block encasing the victim and then it falling down the pit.
Now for the weird parts.
Most of the time my victim got encased in the cooling magma, but sadly the obsidian stuck itself to the floors on which the wall bridges (or floodgates) were located, which makes for other weird stuff seeing that its located INSIDE the bridge located there, thus if you retract the bridge you can see the obsidian block, if you extract the bridge IT EXTRACTS INSIDE THE OBSIDIAN BLOCK!
Now for the extremely weird parts, sometimes, more then often, my victim would SURVIVE, which would normally be entirely impossible seeing that the entrychamber where the victim resides is exactly as big as the magma room above it and the water beneath it. Thus, somehow, someway, my victim manages to sometimes escape being a) charred to death from the magma falling ontop of his head, b) falling into a watery doom underneath him, c) falling into the pit beneath the watery doom, d) falling at all and in fact manage to transport himself from between the soon to be obsidionized magma and water ON TOP OF THE THEN FORMED OBSIDIAN BLOCK!!! The best of it all? Not a single scratch, like some magma dodging, wall jumping ninja dwarf. Ninja Gaiden Fortress.