I as playing In a SSP world when one night, I found a chicken Jockey. Said chicken came up to me but stopped about a block short of attacking me, and would fallow me wherever I decided to go. I then realized I had a few seeds in my hand, and it seems likely the chicken AI was overriding the zombie's.
Everything else seems to work fine, the zombie hurts me if I get too close to the chicken and if I don't have seeds in my hand the zombie attacks.
I am not certain if this was maintained as a feature, and if so, I apologize for taking up your time.
Though this issue was tagged as Works as intended, the chicken AI no longer overrides the zombie's in current versions.