We can't really do it any other way. If it is written in the forum, it's public knowledge. Obviously you're, as players, free to accept any rules or limitations set by other players, and thus "not know about the secret proceedings". But if it's in the forum, someone will have heard and gleefully passed the knowledge to everyone present.
The S&C is a massive undertaking, with servants, valets, squires, adjudants, soldiers and court aides all over the place. That is why you have private rooms to talk in, which can be done through e-mail.
If you want it secret, do it by e-mail.