Time to weaponize FB blood!
This sounds like an exciting prospect. First, You have to find a way to bring the FB up to the surface, trap it in a room somewhere, and kill it, preferably with crossbows from behind fortifications. Then when a siege or ambush comes, lock up your main entrance, and instead of forcing the invaders down a trap lined corridor, force them them through the pool of toxic blood. It would probably be best to build a roof over your little blood chamber, to keep the rain from washing the blood away.
The two problems I see with this are goblins tracking toxic blood out of the blood chamber, and dwarves running through the blood chamber to grab goblin weapons. The first problem could be solved by locking the goblins in the chamber until they expire from the syndrome, and the second one could be stopped by setting orders to forbid enemy equipment on death, though the piles of discarded equipment may still bug some people.
I don't have DF installed right now and I'm waiting on the healthcare fixes before I start playing again, but I hope someone tries this out and can report on the results. What do you all think?