Technically, no problem, render it once to a pbuffer, use that pbuffer as the source for the screen based render and redraw the screen then copy it to a texture as a source for the next frame, easy.
Or it would be, but the texture needs to be a power of two and the window size isnt. So, the logical solution is to introduce another pbuffer into the mix and use that as the destination of the final draw and draw the final scene out as a texture quad stretched over the screen.
At which point we get into the wonderfull area of sharing things between contexts and I start swearing and cursing the name of the person who thought that a pbuffer would be a good render-to-texture solution...
Ofcourse, the framebuffer_object extension would solve all my problems and make all this code like 4billion times easier to work on, but has it been implimented by ATI yet? has it hell [sad]
So, time to ignore this problem for a while and work on something else..