Using a game controller will now count as “activity”, stopping the system from automatically going to sleep or locking the screen. (Yelsin Sepulveda, KDE bug #328987)
Note that this also comes with mouse emulation (as well as arrow keys and Enter/Esc/Meta/etc.) as long as no other app is using the game controller. Thanks Yelsin and GSoC! (Hardcoded button assignments, for now.)