The Leopard voice is Lelouch (and I'm quite certain it was intended to be). The only difference is the idiotic dialogue and the faint echo, not the pitch and the tone.

"Doing comedic roles," and "being able to sound like something between Lelouch and Watanuki," are two different things. He's already done plenty of comedies. He would never fit well in this one, the character simply isn't written for either kind of voice that he is capable of doing.