Matthew,
As an owner of a Rally on a vintage bike I can tell you in no uncertain terms that as poorly as a rally works, there is little reason to even keep one, much less go through the trouble of modifying one. In The Singing Chain there is reference to testing five long cage derailleurs of the time and the Rally came in dead last. They were not one of Campy's more successful attempts. Rather like their mountain bike stuff.
Tom Sanders
Lansing, Mi