I was thinking if it would be feasible to implement an OpenCV-style facial recognition script that could require sustained attention from the user to preserve an open session on the computer. If that user stops watching the screen or is physically separated from the view of the camera, then the locking operation would take place automatically.

I'd be curious from a legal perspective if that would constitute obstruction if you always used it, by analogy to having a data destruction policy in place before the discovery phase of a lawsuit.