To clarify, my understanding of the rules is that metagaming is when information or actions from outside of the game that are *not available to all members of the game* are used to impact the game.
In this specific situation I was referring to the fact that there has never been, to my knowledge, a decision on whether changing play-style on a sitters identity is allowed. And I think this decision rests on the question of: Is a sitter expected to play the country to the best of their abilities or play to the best of their abilities in the bounds of the alliances detailed by the original member. In the case of someone temporarily taking over a game I would expect the initial member to leave instructions and for the sitter to read past press to mimic the members play-style to the best of their capacity. Hence if another member in the game starts changing their interactions to match the sitters past play-style, and not the play-style of the initial players instructions you would have a negative impact to the game due solely to information that is outside the game (the sitters play-style).
Additionally, since the initial member and the sitter are NOT required within the rules to tell any or all members that there is a sitter in the game, there is information in play that may or may not be available to all members equally. Which is where the line between allowable and non-allowable metagaming was drawn in the past. Per the conversation you referenced earlier. So yes, a ruling by the mods probably needs to be made, and I expect it to be something along the lines of:
If the answer to the earlier question is to the best of the sitters ability then:
Members/sitters are required to announce their presence in a non-anon game in global press and the ending of the sitting period. As well as, members/sitters are not allowed to announce their identity in an anonymous game, but must announce there is a sitter
If the answer is the sitter should follow the guidelines left to them then:
There should be no identity revealed in any case about who the sitter is, merely that there is one since anyone should be able to follow instructions.
Since that would remove the "equal" factor/borderline situation. However, I have no idea what the actual decision/if any will be.