As the woman who choreographs the Oscar show, Debbie Allen recently led L.A.'s most fiercely competitive annual audition: More than 400 limber men and women showed up at the ABC Rehearsal Hall to vie for one of the show's 28 openings. Allen's style was direct and sassy, because she's been in their shoes. ''You know, honey,'' she said, welcoming one batch of hopefuls, ''they used to cut me all the time. I remember when I went out for Chorus Line. They didn't even let me dance.'' One who made the grade is Danté Henderson, 21. A theater arts student at Cal State, Los Angeles, he supports himself by teaching dance to children in a local anti-gang program. ''When they made the first cut, it was mostly shorter people. So I thought, maybe I have a chance,'' the 6 foot 3 inch hoofer recalls. When Allen ultimately selected him, he says, ''My heart kind of like jumped.''