Is there an API call that will tell you whether the RCA video output is currently in use?
I have noticed that graphics_get_display_size() always returns the full HDMI framebuffer size (typically 1920x1080) - it does not appear to be affected by the the given display ID or by what port is currently active.
If the app knows this then it can render just the visible portion (720x480?), otherwise you only see the top-left corner and much of the rendering work is wasted.
I'm guessing it must be possible because the console adjusts its size to fit the screen whichever output is used.