Who's going to be making that argument? I'm pretty sure that the SEC (or DOJ) could unmask a corporate officer/director in certain circumstances. But the question wasn't aimed at the practicality of such a regulation...it was: "What regulations govern a public company officer or director's anonymous use of a public message board for promotional purposes?"
If there aren't any there aren't any. If there are I'd like to know what they are...that's the question.