Mmm, magma showers.
I like the quick flush with rapid drain that you describe. Very simple to build, though you eventually need to worry about how you're draining it or how you're refilling it. Either you're dumping to the caverns and refilling from a caldera, or you're pumping to recycle it -- and moving a fairly large volume if you want to sluice it through you're entire kill-hall.
There was a mist generator someone posed a long time back, which was basically a closed circuit of pumps in the ceiling z+1 over your room. it would drop water into the room below, and immediately suck it back out before it could splash or evaporate. This would leave some mud and some mist behind. With magma, you could use it to magma-coat anything in one square at a time. It would not be 100%, but it should light some gobbers on fire for sure - probably breaking the seige and sending survivors running.
My current favorite is the magma mist generator, with impulse driven mine carts. The design I'm futzing with now is the traditional "drop the cart into the magma" design, but I intend to try a "skip it across the surface" type next... as this requires fewer moving carts and less track space overall. I'm not sure if this will work well though -- I have not experimented yet.
I kept trying the "cast an obsidian block and drop it" design, but it is just too damn fiddly when there are more efficient options available. Which is a shame, because I really like the idea of obsidian foundry and the big plop.