Producers of “End of the Rainbow” instituted a strict rule to ban well-wishers from heading backstage after the show’s Broadway premiere at the Belasco Theatre on Monday, so the cast could promptly get to a black-tie after-party at the Plaza. But showbiz veteran Elaine Stritch, 87, is not one to be stopped. Formidable actress and singer Stritch burst through a barricade to give star Tracie Bennett — who plays Judy Garland in the play about her final days — some precious opening-night advice. “I need you to know how wonderful you are, but listen to me, and listen carefully: I don’t do matinees,” Stritch was overheard telling her. “Your producers will kill me . . . but you should not be doing eight shows a week!” Despite the sterling advice, a production rep told us the drama by British playwright Peter Quilter will go on as planned. He said, “Tracie will continue performing eight shows each week. She did more than 300 performances in the UK and never missed a single [one].”