For his current production — a present-day take on the van Eycks’ Ghent Altarpiece — Milo Rau, artistic director of NTGent in Belgium, placed a classified ad looking for jihadists. He got no takers, but he did get hate mail and an international media furor. Once the piece got onstage, though, the praise was warm and widespread — the usual pattern with Rau’s work. “I’ve had scandals before a premiere,” he says, “but never afterward.”