Overlay text over external fullscreen in OpenGL

Hi all.

I’m looking for a method of overwriting the final rendering output in a third party aplication that works at full-screen in OpenGL, but without hooking or hijack that aplication (if it’s possible).

The overlay doesn’t need to have alpha, i’m happy enought if i can overlay the text into a simple black box. (But if that box has a bitmap on it, much better).

I doesn’t find any good open source example, but FRAPS, Xfire, or GameOverlay.com seems to use very complex methods to overlay any OpenGL window in full-screen mode. (But i suspect that they use some hooking).

How can i find a good book, or open source example of Overlays in Windows’s OpenGL external aplications?.

Is possible to overwrite the backbuffer at some fixed coordinates with my desired overlay, but avoiding blinks?.
Any good SDK or public document about how to work with the backbuffer?.

Thanks in advance!.