I'm not talking about them not attacking Lucian in-game, I'm actually really talking about lore.
I expect Yorick to prevent his Mist Walkers from attacking Lucian, if the two ever encountered.
Because, well, Yorick could probably see that they both share similar interest in wanting to end undeath (mainly on the Isles), and Lucian is still a living human, not yet corrupted by the Mist, so Yorick would probably even want to help him, just as he helped the guy in his new short story, Last Rites.
Although, the big problem here would be that Lucian will probably try to kill Yorick on the spot, with no desire to hear Yorick try and explain himself or his Mist Walkers. Only then I'd really expect Yorick to act in self-defense and send his Walkers onto Lucian. :P
I could be pretty wrong about that last part though.