oh, that’s a good one… First you need to get the person’s height. You can’t put anything on the person, so maybe a distance sensor above the area facing down. Get the distance when no one is around, then place someone in place, get distance then subtract for the difference… But there are variables that come into play; adjustable pivot point for the mic, is the person going to move? This idea will not put the mic right in front of the mouth, but can get fairly close. Maybe have the person adjust the mic at the pivot point?
Only other inexpensive way is to just have it motorized with a second person with a remote.