Showing/hiding (or activating/deactivating?) overlay based on game being played?

Extensions
3

Pretty sure you can show/hide the UI based on the game. You can get the currently played game with the onContext callback from the twitch helper js.

window.Twitch.ext.onContext(function (context, contextFields) {
    //console.log(context.game);
});

Then you can hide the UI with display: none or however you choose.