There’s no need for a specific class of meta-mods. Just use the same system as Slashdot: periodically throw a random moderator’s decision in front of another mod. Ask them if the decision was fair. A portion of the bad decisions will get reported and screened.

If all mods are routinely screening each other’s work, everyone has an incentive to be fair. Biased mod decisions are thus a risk to your continued employment because you know your work is spot-checked.