Switch between monitors using camera
You may be able to craft your own solution with a combination of scripting software and assistive hardware.

Assistive hardware exists to track head movement. Most assistive hardware can be used to provide an input that can then be scripted via AppleScript or shell script.

A few examples are linked below. I have no experience with these tools but your goal appears possible.

Headmouse


  HeadMouse® Extreme replaces the standard computer mouse for people who cannot use or have limited use of their hands when controlling a computer or alternative and augmentative communication (AAC) device. The HeadMouse translates natural movements of a user's head into directly proportional movements of the computer mouse pointer, so as the user moves their head the mouse pointer on the screen also moves.




SmartNav


  The SmartNav 4 AT is a total hands-free mouse solution. With built-in Dwell Clicking Software and a free On-screen Keyboard created by Lake Software. The AT package is used successfully by people with ALS, spinal cord injuries, muscular dystrophy, and other special needs.




One Switch

One Switch sell a wide range of assistive hardware and link to associated software.


  The following list details a number of different styles of head, mouth and eye operated controllers. Most of these are for PCs and Apple computers, but there are alternatives for games consoles too. Some of these devices are very expensive, so it is always worth trying to track down some way of trying before you buy.