I'm really going to implement my idea. With being able to bring huge quantities of stone there much faster, legendary siege operators can cause a rain of stone to fall. Catapulted stone may not do much damage, plus the stone is destroyed, but this way you can recycle the stone and stones falling some z levels should cause some damage. and with a crew of legendaries, firing rate should be great. The only thing to worry about is flyers that don't get hit by falling objects.
This could have some interesting applications against fliers, actually. If you're firing catapults so that the stones drop down onto grounded enemies through small vertical tubes or somesuch, flying enemies would probably path up the tubes and get hit by falling rocks, or by the stone still in the air from the catapaults. Plus, if the vertical tubes are long enough and your catapault count is sufficient, they'll get killed in mid-air most of the time and their (presumably heavyish) corpses will fall back onto enemies.
I did some testing in the arena mode: killing giant cave swallows above goblins in a 1x1 chamber that guaranteed they'd get hit by the falling corpses*). The goblins were armored with a copper helm, breastplate, and shield, had a silver maul, and competent fighter/shield/armor/dodging/hammerman skills. If the goblins' level is considered level 0, the swallows were killed (adamantine axe to the head) and dropped from level 4. Combat log samples are below. It looks like this tactic might prove viable if the fliers could be killed reliably on the way up the tubes.
I am uncertain as to whether corpses missing limbs will do less damage. Does their weight change on limb removal? If so, keeping them intact through the use of blunt attacks would be to one's benefit (unless peppering goblins with disembodied fingers and toes is somehow useful).
You hack Giant Cave Swallow 4 in the head [etc...]
Giant Cave Swallow 4 has been struck down. [it falls]
The Giant Cave Swallow 4's corpse strikes Goblin 3 in the lower body, bruising the muscle and bruising the guts through the small copper breastplate!
The Goblin 3 slams into an obstacle!
Goblin 3's left lower leg takes the full force of the impact, bruising the muscle!
Goblin 3's left upper leg takes the full force of the impact, bruising the muscle!
it Goblin 3 in the right true ribs, but the attack is deflected by Goblin 3's small copper helm!
it Goblin 3 in the right true ribs, but the attack is deflected by Goblin 3's small copper breastplate!
You hack Giant Cave Swallow 5 in the head [etc...]
Giant Cave Swallow 5 has been struck down. [it falls]
The Giant Cave Swallow 5's corpse strikes Goblin 3 in the left lower leg, shattering the bone!
Goblin 3's right lower arm skids along the ground, jamming the bone through the right elbow's muscle and shattering the right elbow's bone!
The Goblin 3 loses hold of the silver maul.
The Goblin 3 slams into an obstacle!
The Goblin 3 gives in to pain.
You hack Giant Cave Swallow 6 in the head [etc...]
Giant Cave Swallow 6 has been struck down. [it falls]
The Giant Cave Swallow 6's corpse strikes Goblin 3 in the right lower leg, shattering the bone!
Goblin 3's right lower leg skids along the ground, shattering the bone!
The Goblin 3 slams into an obstacle!
*thus, dodging was not factored into the test-- I was merely assessing the damage a falling cave swallow corpse could cause to a traditionally-armored goblin. It seems that armor will stop most major falling-corpse damage (lung-bruising is still possible, as evidenced by the bruising to the armored lower body by GCS 4). The limbs, however, are still fair game.
EDIT: Also, here's a semi-related idea: have a multi-leveled fortress entrance, with a bridge as each level's floor. Guide sieges into the top floor, killing them in whatever manner you'd like, and when the next siege comes along, lead them into the floor below and drop the first siege's remains onto them through the use of the bridge-floors. You can snowball this effect again and again for greater damage each time, eventually collecting the loot on the bottom floor.