My daughter invented something similar. It turns out that a whiteboard marker will draw anywhere on a Linux desktop screen, and it's also completely cross platform.
This used to be helpful for those games that didn't give you a crosshair.
I've actually done this a few times before - it works really well until you write on a screen with a particular kind of high gloss anti-reflective coating, which it damages, and leaves you with visible lines even after you wipe the ink away. Not recommended!
She found a hardware solution to a software problem.
I tried this method, took a screenshot but it didn't appear on the screenshot. Do I need to use a permanent marker instead?