I wouldn’t entrust such an important display to some hacky solution a bunch of Lemmy users can come up with. Set up a normal kiosk environment and do your work on a second machine. You can ssh or even Sunshine/Moonlight into the gaming machine if you have to do something there.

But you don’t want something a potential customer does on the game to crash your work or vice versa. Or even worse something goes wrong and the customers can suddenly see your work on their screen. Best to separate those things as much as possible.