I didn't get what you're trying to do. sure I dunno what ya mean by 'all the FB data is there'. there're some fb data that could be read by imf_disp on render time but for that you'll need probably the integration library. also that fb stuff is generally deleted on the end of the rendering.
but probably you could achive something like you're after by using mr passes. you'll save the main pass with geometry and all of that. then you hide all the geometry and merge the pass you saved with the current scene, where you have just a lens shader to switch fb. fbs are saved with passes.