You can add a draw callback, for instance on the DisplayRegion, which allows your own code to be called after the frame has been drawn. See this thread for more.
Of course, Panda already provides a mechanism to extract the frame data each frame via glReadPixels (or the equivalent DirectX call), using the render-to-texture mechanism. But if you are interfacing with a library that expects to make the glReadPixels call itself, then you will have to use the clumsier callback mechanism.
David