Does your derailleur have any aluminum on it? I don't really know or (maybe I shouldn't admit this) much care about old Campy stuff. But keep in mind that OA will eat up aluminum. When I have to de-rust a derailleur or other assembly than contains both steel and aluminum parts, I dip it overnight in a water-based product called Evapo-Rust, which dissolves rust extremely well but does no harm to aluminum. It's kind of spendy at about $8 a quart, I think, but you can use it over and over. I've been using the same quart for about a year now. If you can't find it locally, I think you can order it through Amazon. That's where I got mine.
I would love to see what a vat of this stuff would to to a rusty frame. It would be expensive to start, but might even be worth it if you treated a lot of frames.