That sounds like a huge hassle to be honest. My android phone had a feature that made the screen stay on as long as I was looking at it - it was a disaster. It just did not work properly. I ended up deactivating all that crap and just set the screen to never go off, unless I explicitly press the button.
I would imagine that if you were up to something like this, the laptop in question wouldn't be your daily driver.
In which case you would likely want to physically disable inputs and sensors like cameras, mics, location hardware, etc.
I seem to also recall once reading that it was good measure to use a laptop with a removable battery, having actually removed the battery and only being powered by your power cord.
Or so I've heard.
isn't facial recognition easily defeated with a picture of the person's face?
that said i concur, the proliferation of built-in cameras should spur us to adopt biometrics such as facial recognition, but it's gotta be better than picking out a few points and doing a simple match like most CV facial recognition systems do. some sort of challenge-response maybe, like "wink your left eye" or "blow a kiss", anything that a simple, static picture couldn't defeat.
but i concur on the larger paradigm - the hardware is common enough to force us to consider biometrics as commonplace.
Given the proliferation of comments about how to avoid linking meatspace and digital personas, having a laptop with a camera constantly filming your face, and a piece of software designed to recognise only your face stored next to your incriminating digital files seems like an escalation of risk to me.
Use a laptop with a camera. Have the camera require that your face is in the picture to avoid activating the screen saver.
My cell phone already does this, so it's got to be easier with a full powered laptop.