Yeah, the GPU process can do this regardless due to the way Windows forces any accellerated GPU app to work. I'm not sure if you dug into this before, but the GPU does run as low-integrity for UIPI purposes on the interactive desktop (lower for other object access). Unfortunately, the desktop window, screen, and display state are still exposed at that level. We're digging into AppContainers to see if tighter restrictions are possible on Win8, but usage is still so low that it's not a priority.
Sorry I focused solely on Windows, but to my knowledge all platforms we support have similar architectural niggles in terms of the GPU process being able to read anything that's displayed.
