An idea I had for working Humanity into Multiplayer would be to take advantage of the fact that each player has their own screen.
Let's say that you're walking down a street and a group of party revellers spill out of a night club, happy, jolly and buzzed to the hills.
A player with high/max Humanity would see a group of happy revellers.
As you work down the Humanity scale, the perception of the group blurs to drunk, rowdy and a bit mouthy; then on to raucous and openly abusive;
Until you get to the lowest (playable) Humanity character, whose twisted and tortured view of the world perceives a bunch of hostile gangers bursting out, weapons drawn and spoiling for a fight.
/discuss?