I built this mainly to test the effects of cave-in dust.
Veshstal, "Dustbuster"
Floor 0
Floor -1
Floor -2
On Floor 0, those are 1x1 bridges to either side of the center gap, currently covered by 1/7 water or magma. The X's to the north and south are floodgates, made to float without a floor by constructing a wall on the z-level below to make a temporary floor. This allows the fluid to be funneled into the center tile of the shaft without providing a support for the obsidian block.
Both water and magma are pressurized, but have limited flow from their source. The diagonal choke may be unnecessary. If you have a lot of flow, it might risk pushing one of the fluids across the gap and jamming it.
Most of the time the block would form at level -1 (or at least that's where it zooms to). I never saw it form at level 0.
My shaft went around the magma sea, straight to the semi-molten rock. (expose the magma flow tiles by digging down staircases and then channeling them out.) For some reason it started filling with magma some time after I finished digging it out, possibly because it was in the same embark square as my magma pipe.
The results from my test chamber:
Sometimes the horse wouldn't show up on "view". Going to the units list and zooming to its position showed it to be above the floor. So it would appear that cave-in dust causes injury by flinging units upwards, which I think even drawbridges can't do.
On using this against HFS, you could line the sides of the shaft with 1x1 raised drawbridges made to float by the wall-removal method. This will force them to fly up directly in the path of the block (unless they can destroy drawbridges, use magma-safe materials in case they can be ruined by superheated ones).
Since obsidian is technically a glass, maybe this could be called a glass cannon