I am concerned about modeling nonexistence as zero or infinitely negative utility. That sort of thing leads to disincentivizing the utility function in circumstances where death is likely. Harry in HPMOR, for example, doesn't want his parents to be tortured regardless of whether he's dead, such that he is willing to take on an increased risk of death to ensure that such will not happen, and I think the same invariance should hold true for FAI. That is not to say that it should be susceptible to blackmail; Harry ensured his parents' safety with a decidedly detrimental effect on his opponents.