Can one of the nation's great musicians cut through the fog of a D.C. rush hour? By Gene Weingarten Washington Post Staff Writer Sunday, April 8, 2007  HE EMERGED FROM THE METRO AT THE L'ENFANT PLAZA STATION AND POSITIONED HIMSELF AGAINST A WALL BESIDE A TRASH BASKET. By most measures, he was nondescript: a youngish white man in jeans, a long-sleeved T-shirt and a Washington Nationals ba…