When manly men like Samuel L. Jackson and George Clooney drop trou for kilts, NYC's Metropolitan Museum of Art's Costume Institute takes note. And its new exhibition, ''Bravehearts: Men in Skirts,'' which runs until February, doesn't skirt the issues. ''It explores the changing ideals of masculinity,'' says spokesperson Bernice Kwok-Gabel. Hopes participant designer Howie Nicholsby: ''One day men can choose between trousers and a [skirt]. Just like women.'' But can they deal with panty lines?