HE’S A DAPPER DIDDY, and Sean Combs expects everyone to follow in his Bruno Maglis. Invites to his and record exec Guy Oseary’s MTV after-party at Manhattan’s Cipriani included a strict ”party policy.” ”Pull out the flyest s — – in your closet,” the edict advised, ”or have your stylist pull something for you.” The rapper/clothing designer discouraged jeans, suggested waxing for ladies and shaves for men, and urged invitees not to ”disturb the sexy.” Guests Mike Myers, Nelly, Mary J. Blige, and Missy Elliott made it through the rigorous checkpoint, but Ethan Hawke and Avril Lavigne were reportedly among those turned away. So what’s with his going Emily Post-al? A Sean John rep says: ”It was meant to celebrate the awards, but obviously the dress code was true to his form.”