I can't see that happening. Ever.
Besides, if it did, I would feel very sorry for Roy. Messing around with the Joker's punching bag is a sure way to get killed (of course, in regards to the Joker, it's easy to get killed).
And, of course, Harley is evil. She enjoy's killing, she's addicted to it. She doesn't even need the Joker pulling her strings anymore.