A while back I decided to roll a new warlock on live, so I was going about my day and discovered a Hordie, flagged, on the Ratchet dock. I figured "why not?" and sent the Voidwalker after him.
To my surprise, the Voidwalker was not attacked by the guards.
The Horde player who was being attacked began to retaliate and was promptly slaughtered by said guards.
No honor kill, but definitely fun to do!
Notes: I did not attempt a second time, the bastard got away. I have not tried this with any other pets. If it doesn't work for you then I apologize for the misinformation, I probably got lucky. If it does work, then happy hunting