> It's possible some hardware already does overlays, given that the talk I linked above was 2 years ago. I haven't researched this carefully.

All hardware does some overlays, though not a large number. (You can see Intel GPU overlay setup at [1].) One scanout overlay is already in use on all major OS's, to draw the mouse cursor.

[1]: https://github.com/torvalds/linux/blob/e60e1ee60630cafef5e43...