I don't see the problem with using the chalk on the rail as a reference point. How is it any different than using something in the background? Also, how can this be enforced? Anybody could claim the opposing player is "marking" after each time they chalk....
I think the rules only say you cannot mark the actual playing surface....
I agree with you, I don't see a problem either. I have seen some rather sporty novice players use chalk, moisture from their finger, and various other forms to mark the rail and even the cloth on the playing surface. Guys like Archer, SVB, Morris, Duel, Strikland,and others. I was even shown several ways years ago how to mark the rails discretely by some of the players mentioned.