For a more repeatable weapon with the same effect, you could make the floor out of lever-linked floor grates (as already suggested) and replicate the dust effect by having blocks of obsidian linked to supports. How, you ask? Well here I present the Mk1 Obsidian Bomb Dropper:
side view:
w f f f f f f f w
w S x
x o o o o o w
g g g g g g g g g
Key o = obsidian, S = support f = constructed floor w = constructed wall g = magma safe retracting bridge x = floodgate
The bridge acts as a "safety" (and should have its own lever) When the lever is pulled the obsidian falls, smashing through the floor and horribly splattering anything beneath it, without risking your military dwarves. Then the bridge can be extended and magma and water introduced through the floodgates to regenerate the obsidian. Have miners remove obsidian from the sides, build a new support, and presto! a reloading obsidian dropper. Soften the enemy up with an aerial bombardment and then send in the paras!
I'm not sure if its quite as dwarfy as the falling barracks option though...
/edit: Also I think it may be possible to eliminate the mining step by putting a row of floodgates around the edge (where you want a gap between obsidian and wall). These can be closed when the obsidian is setting, then opened as part of the firing sequence (idk if that's strictly necessary). So the only manual part of the reload procedure is rigging up the support, and if the bridge on its own can support the obsidian (I don't see why not) then even that is not necessary, and everything is lever-controlled. Hell, a bit of cunning with fluids and pressure plates and everything could be automated (except the actual trigger being automatic probably isn't desirable!)
Semi-automatic obsidian bomber anyone? Once I get reliable access to magma, I'll build a prototype to find out if it actually works...