I believe what the poster you were replying to was asking was if you add a definition of "preferred form" does it violate the no additional restrictions clause. Although in general your reply is accurate it doesn't appear to deal with that specific situation and the OP may feel you didn't answer their question.
I'd argue that preferred form is already defined by the license although I'm sure some will disagree based on the previous threads. From the minimal amount of legalese training I've had it's apparent to me that preferred form is already defined in the license, although indirectly. Whether clarifying some additional description of preferred form violates the no additional clauses would be as you say up to the courts of whatever jurisdiction the case is brought to trial. It would take some research on precedents in the US to draw any kind of conclusion on what the ruling would be in the US district courts.
Personally I believe you have a pretty good chance of the courts accepting that additional clarification as long as the licensee is aware of the clarification at the time of license. The key to that would probably adding the definition to the license but the FSF would probably tell you to stop calling it GPL if you did so. Therein lies the major complications of such an action.