AN exclusive formal ball will mark Halloween in San Francisco this week. In couturier gowns and elaborately confected masquerades, the couples will whisk around the floor until 2 a.m., while judges award prizes for the best costumes and the participants elect an “Empress.” By then the swirling belles will sound more and more deep-voiced, and in the early morning hours dark stubble will sprout irrepressibly through their Pan-Cake Make-Up.