This isn't an answer to your question, but rather a follow-up if you find an answer.
Would it accomplish your goal? I assume what you're trying to do is force developers to enter a log message; to encourage those lazy developers to post meaningful log messages instead of empty ones.
So, thinking as a lazy developer, what would they do when confronted with a required log message? I would bet each file eventually gets populated with logs like: "fixed bugs", "quick fix", or (my personal favorite: a random characters) "isabvsuibv". Never underestimate the tenacity of the lazy. I've actually seen people do this before, and there's no way for CVS, or any filtering program, to determine whether the log message is meaningful or gibberish. Even if you could, there's no guarantee the developer actually remembered all the edits made.
I'm not trying to say what you want won't help, but if the developers don't have good habits, it's going to be a serious uphill battle to get them to change. And forcing them to do something they don't want usually makes them more resistant.