So, my FB-killing cave-in trap worked - though there were a few problems. Everything went well at first - with careful micromanaging, I was even able to get the miner who opened the cavern back through the airlock and into the fort. The FB ran in to break stuff, I closed the outer door to trap it, then pulled the red lever....it turns out that I very stupidly forgot to check underneath the cave-in room, so when the thing went off, it blew a massive hole through 2 or 3 z-levels into another part of the cavern. The FB was very dead, though.
I reconstructed the floor in the room and set about re-arming the trap for when the next one inevitably showed up, but one of my masons decided to build directly off of a bridge when rebuilding the falling ceiling, causing another small cave-in. The mason that caused the cave-in slammed into a wall and died, but he was only level 5, so I just idiot-proofed a few build orders and moved on. What I didn't notice, though, was that my legendary +5 mason was also in the cave-in room when the floor fell, and he must have staggered through the new opening in the floor into the cavern and lied broken and unable to path back into the fort until he died of dehydration. I was kind of bummed out about that. I probably would have savescummed to try and rescue him if I'd had a saved game recent enough that I wouldn't have lost 45 minutes of play on the fort.
Oh well. Masons are relatively easy to train. Rock blocks, coming up.