In a way, it was rather smart. Lelouch can't pretend not to know Nunally without hurting her - something he'd never do. He could pretend to talk to her as if she were somebody he had just met, though. I don't want to elaborate on this though, so if you don't understand what I mean by that, don't ask.
However, I don't think it was the smartest move either. Since he cares for his sister more than anything, one would think she'd have a pretty decent chance of bringing his memories back right then-and-there. Lelouch could also act like that happened, though it wouldn't gain him anything unless Suz got into trouble.
My guess is that Rollie's Geass comes into play. teehe.