So, no magma (in the simplified, non-over-engineered version). But fairly easy to light dogs. That you can make sure can't go in any direction that you don't want them to go. That are being fired in a general direction from a lever. That can be set to be repeatedly pulled. To ensure that eventually the dogs WILL get over the moat, or die in the process. That after getting over the moat will always attack enemies.
Can't these same wardogs (or any pets that are under the bridgeapulting/atomsmashing limit) be pastured to an area? Or if you wish, make a one tile meeting area for them? And taking out 66-75% of available directions of their trajectory with walls, with them auto returning to the area to be 'pulted again, won't it be a little flaming-doggy-cannon? Crispy doggy death for a few of them on return journeys to the flinging area is definate, but the few that make it may do wonders. Bigger war animals may even take a little longer to go crispy, therefore allowing more chances of on-target bridgepulting. Perhaps it is the whole trainable egg-layer geometric scaling issue finally weaponised fully? Flaming-cave-crocodiles or merely a friendly pink-poodle-of-doom?
It's way off on a tangent from the OP, but making dwarves fly over a ditch and into battle is only one step from setting up a repeatable system in which you light your live-stock with lava and fire them at the enemy, knowing that not only will they attack the enemies, the whole system probably will burn your whole damn fortress down. And it will still be your most reliable weapon.
"The !wardog! has latched onto the goblin lasher's upper right leg."
"The !wardog! shakes the goblin lasher furiously by the upper right leg."
"The goblin lasher's nether regions disappear in a puff of smoke."
"The goblin lasher gives into the pain."
"The goblin lasher has been struck down."