Hmm. An interesting prospect indeed. While Arms Slave still plans to join the Black King simply for the novelty of it, if you could somehow get someone to recruit Arms Slave once the IC comes up, I'm sure he'll be willing to join the Ivory Mask for all the damages. 

Still, as long as Arms Slave finally gets to do some killing himself, I'm sure he won't mind offering his body for the White King. Sure he'll put up a fight, but he hasn't yet grown out his fangs. Also rereading White King's skills, I think Chain-Catch could be useful. A good gap closer to ensure that no one escapes her. Even Chain of Hate is only useful if the enemy is within your melee range, while Chain-Catch can go as far as the eye can see. Which for Arms Slave isn't really that much, but who knows what the White King will do with it. Though Surviving Suffering will be important for the late game as it scales with level; I chose that skill specifically to counter single-hit nukes and ensure that I never have to play rocket tag with the enemy. If Arms Slave could get his hands on equipment that heals him as he fights, he'd be damn near invincible. Even if it may take him hours to grind your health away.