Got the forgotten beast silk farm finished, and harvesting silk. Sure it isn't exactly valuable, but you sure can't beat taking advantage of a monstrous, terrifying beast for some nice silk clothes.
Z-0
This is the repeater for the drawbridge a level lower, every 100 steps or so the bridge retracts and dumps the silk down a level for safe collection. The hole behind the wall goes down to where the bait is dumped, had a problem with dwarves running by the hole getting spooked so I went up a level and put in another hole directly above, preventing the silly buggers from being scared of the caged beasty.
Z -1
Here you see the collection bridge that retracts, the trap for the beast consisting of two raising bridges (the beast stands on one, it moved as the lever was pulled) and the door meant to keep him busy while the lever was pulled. The bait is a poult, dumped there from above.
Z -2
This level catches the silk after being dumped once, and every so often the lever on z-0 is pulled to drop it one level further. This step is needed to prevent "Urist McWeaver is struck in the head by the falling Forgotten Beast Silk, shattering the skull and tearing apart the brain!" messages. The side room collects any extra silk that happens to fall in the holes that prevent the bait from moving around too much.
Z -3
This is the final collection / harvesting area. The door is locked when the lever is pulled, keeping everyone safe from the deadly falling silk. Weavers then are allowed in to pick up the silk and turn them into threads, while master weavers produce fine masterwork cloth from the raw thread.
Currently have about 40 forgotten beast silk cloth and around 1500 uncollected webs/threads sitting around, after maybe a half season of collection.