This translates into an informal principle of negotiations: Be willing to accept unfair bargains, but only if (you make it clear) both sides are doing worse than what you consider to be a fair bargain.

That's clever and opens up a lot of extra scope for 'imperfect' cooperation, without any exploitation problem. I notice that this matches my 'fairness' instincts and some of my practice while playing strategy games. Unfortunately I don't recall reading the principle formally specified anywhere.