> I'd love it if someone found a solution for putting a tiny magnet in exactly the right spot to force-disable the microphone while the lid is open. :)

It's the same sensor that's used to detect if the lid is open in general. Triggering it will prevent the display from turning on, and will make the computer go to sleep if it isn't running in "clamshell mode" (running on AC power with an external display attached).