I just added a new sub-section to the Posting section, here's the wording I'm using if anyone has any suggestions or corrections.

Code Phrases: If you are allowed to NB other players, you may seek to set up code phrases that can be innocuously used to send messages, however, you may only use correspondence legally transmitted in the current game for this. You could not, for instance, use codes from previous games where you could speak to a player to send them messages in the current game, nor otherwise non-public correspondence, like telling them to 'Investigate the player whose first name starts with the same letter as the first word in our NB on date X'. You could however tell someone during your legal night phase NBs 'If I title a message What should we do next? vote for the first player I name in that message.' This rule prevents players who have extensive prior contact with many players from gaining an unfair advantage, a player for instance who has moderated a previous game will have exchanged private NBs with most players.