The video that is on the site, but buried, doesn't seem to show anything.
This would be cool as a menu bar application that shows up when it detects an external display, sort of like a rear-view mirror for my Mac. I wonder if it would be possible to move things around inside it with the accessibility APIs that window managers use.
BetterDisplay can also do both, as well as a truckload of other things (some free, some paid).