Gonzo,
Rust and/or other imperfections (like pits, dents, wear, etc.) on the shock rod will cause oil leakage. The seal can be easily damaged by anything on the chrome surface. Like Dave mentioned, ride it until it leaks... just don't be too surprised if it happens rather fast. Trust me... I am a suspension engineer (in the auto industry) and deal with shock leakage issues from rod imperfections all of the time.
I recommend sending your shock to someone for a rebuild and get a new shock rod. It will be good as new and cheaper than replacing the entire shock. If you have the proper tools and access to nitrogen gas, you could do the work yourself... but in my opinion it is more cost effective to let a professional (such as LT-Racing) handle the shock rebuild.
By the way, I noticed your other post regarding your seized engine. I wonder if the previous owner left the bike sit around a long time. The presense of rust on the shock rod (which is rare) may be an indication of corrosion on others components such as cylinder wall... leading to premature engine seizure. Just a thought.
