Dumb solution: an FAI could have a sense of justice which downweights the utility function of people who are killing and/or procreating to game their representation in AI's utility function, or something like that do disincentivize it. (It's dumb because I don't know how to operationalize justice; maybe enough people would not cheat and want to punish the cheaters that the FAI would figure that out.)

Also, given what we mostly believe about moral progress, I think defining morality in terms of the CEV of all people who ever lived is probably okay... they'd probably learn to dislike slavery in the AI's simulation of them.