lemme try again. The cable housing runs from the top tube stop to the cable housing stop on the derailleur. The cable passes through and is fixed to the bolt at the bottom. The derailleur is made to work by the housing pushing on the cable stop that is on the derailleur; the cable (in this setup) just provides the equal and opposite force to make the housing do the work. If you could find some other way to secure the cable housing that is needed to route the cable down...and a pulley that works with the bolt and boss at the bottom...then you could use a conventional bottom pull derailleur.