Yes actually, they would cause them to catch fire if exposed for long enough periods (not exactly sure, but certainly 1 burning object across the hall won't cause them any harm.) Your best bet is still probably magma or lignite (lignite needing to be exposed to magma in order to begin burning.) Or an alchemy reaction that produces infinitesimally burning oil for you... Poisonous oil to place along side it that boils and causes massive hemorrhaging...
But do body parts from a burning animal remain burning after the animal has been destroyed? I don't know...
As for my favorite trap, I like to place marksmen behind fortifications along a trap corridor, also having a short dodge-pit trap before meeting my melee troops.
I want to make a labyrinth of various wildlife, mega/semi-megabeasts, titans, and FB's to toss goblins into. Or to unleash upon them after sealing them in a massive empty room via collapsing cast-obsidian walls. One way in, but can you get out?
That would require sacrificial animals to get the beasts back in their labyrinth, or out into the room...
Maybe a wide, deep chasm with balistae and marksmen on the opposite wall as I've heard described on this forum. The path could be narrow and include weapon traps.