Somewhere along the line I considered and rejected the idea of wiring up the limit switches on the scanners and inkjet, thinking it would be too much trouble. Instead I've been moving each of the motors to a reasonable starting position before starting playback. After several near-misses (always keep a hard-wired "motor power off" switch handy!), I'm reconsidering that position... there are simply too many ways for the software to break and potentially cause something to run out of limits. Additionally, re-homing the motors each time is getting pretty old.
In order to mitigate the homing problem, I taught the Arduino to reposition the motors at the end of the song (or when it's stopped by ejecting the floppy). That should make it a little bit easier to run headless (without a laptop driving). I had several more near-misses in the process of programming that function, though, which tells me I should have just wired up the limit switches in the first place! I don't know if any of the scanners or inkjet could physically self-destruct by hitting mechanical limits, but I'd prefer not to find out. The floppies are a little smarter and won't step past their limits.
Got a director lined up, hopefully recording today, woohoo!