This is an old post and probably very out of date, but: I think if you try to define an impartial observer's preferences as whatever selects (C,C) in two other agents' PD, you get inconsistencies very rapidly once you have one of those agents stuck in two Prisoner's Dilemmas at once.

I also don't think we should use euphemisms like 'impartial' for an incredibly partial Cooperation Fetishist that's willing to give up everything else of value (e.g., billions of human lives) to go through the motions of satisfying non-sentient processes like sea slugs or paperclip maximizers.