Works on my iPad 2, but I’m not sure about other devices. Basically the update prompt will appear but the player will still join the game in the background. They can’t play because the prompt is in the way but I imagine this could potentially have a large impact on certain games.
Step 1: Go to a game’s page in safari
Step 2: Play in App
Step 3: The app will launch and begin the join, and prompt you to update. If you don’t do anythinng it will continue to join and then you will be in the game.