Make it easy for developers to detect available screens (internal to the device, AirPlay such as Apple TV, or a screen connected to the device via an HDMI output cable. Then select the screen for drawing. Ideally you could draw independently to multiple screens, but an intermediate compromise would be to select only 1 screen for drawing, but still accept touch input from the device, to use as a controller (maybe you could display a static image on that screen to give the user touch points).