Up until now, I've been focused on hardware (non-firmware) status lights which show outbound cell traffic, outbound wifi traffic, microphone data, and camera data.

You know what? I think I'd like your physical kill so much better. It could cleaner to implement, too. If the manufacturer couldn't offer it, I'd be happy to do business with a modder who could.

An existing exterior button like the iPhone's ring/silent switch seems like an ideal starting point to work a third-party hardware solution around. But given what little free space exists inside these devices, here's hoping that solution is more wiring than components!