On a slightly unrelated note, are siege engines hardcoded?
Siege Ammo isn't....hehehe..
flaming ballistae arrows...oh the possibilities...
Magma Catapult? Must be built over magma channel, with firesafe (necessarily Adamantine?) catapult parts
plus block, tube and screw. Loading == pumping, firing == ballistic magma blob. It not only acts as a standard balistic mass (with intrinsic flight-path damage to hostiles, friendlies, constructions and scenery alike via heat, c.f. ballista-bolts with whatever form of damage they inflict upon things), but on landing is also a 7/7 magma to flow and evaporate as it will[1].
Various operational factors (extended pumping effort, need to maintain if materials aren't fully magma-proof, possibly heatstroke effects on operators) could ensure a degree of balance. And it's probably not possible without tweaking the hardcoding, and probably beyond Toady's tech level intentions, but pretty close to the magma-cannon concept that I see bandied around.
[1] A quick thought occurred to me that in such a setup a strategic set of 1-deep bridges capable of being raised or lowered along the catapult's firing line would let fine tuned 'dropping' of damaging magma-blobs on a lower Z-level in a way that you can't do (for damage purposes, lestwise) with standard catapulted rocks. Unless the constructions can't withstand the flightpath damage, of course.