Yep it attaches to the fence, I don't see how you could design something to slide on two axis and still be as accurate as the positioner system, seems like you would be re-engineering the whole rig so why even buy it in the first place? It would seem a whole lot easier and probably much cheaper to just buy a leigh jig or something else designed for large pieces if that is your primary source of work.
If you do come up with something I would be interested in seeing it though so post up some pics when your done!