Unless you can directly extract a sincere and accurate utility function from the participants' brains, this is vulnerable to exaggeration in the AI programming. Say my optimal amount of X is 6. I could program my AI to want 12 of X, but be willing to back off to 6 in exchange for concessions regarding Y from other AIs that don't want much X.
I had also mentioned this in an earlier comment on another thread. It turns out that this is a standard concern in bargaining theory. See section 11.2 of this review paper.
So, yeah, it's a problem, but it has to be solved anyway in order for AIs to negotiate with each other.
And happy new year to everyone.