But why wouldn't you hide it in the same document that you were sure would not be published, as it isn't normal practice to do so? That way, you would make it readily available to all those who 'need to know', but keep it away from prying eyes.
If I worked in a leaky environment, I'd be tempted to keep advice like 'how to break a legally binding treaty that I'm about to sign up to' verbal, or at least away from something that might possibly be of interest to other parties / the press.
I doubt that kind of thing would appear in the same document though (I've no experience with this, it might...)?
Isn't the simplest explanation that there's something bad / very bad in the legal advice that they want buried for now.