You're familiar with those "#lightboard" systems right: clear glass, side-lit, you write on one side and a camera films from the other to give the illusion of drawing glowing diagrams in air. If not:
https://lightboard.info/
But I am thinking: could you do similar with a virtual camera and a "light pen" for writing in the air? Consider a hollow dry-erase marker with momentary pushbutton that turns on a bright LED + signals to computer "hey, pen's on!" Then the computer finds the bright spot, draws / stores it in an overlay on your video stream, and presto you've got a magic whiteboard pen?
This seems so simple it has to be done already and I just don't know the search terms. Help, anyone?