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For younger Hollywood celebrities, the hippest post-Oscar party of the night wasn’t the Governors Ball or even Irving Lazar’s annual Spago bash. The place to be for more than 1,000 of Hollywood’s hipsters was Le Mondrian. Proceeds from a $500-a-ticket black-tie event--still being tallied Tuesday--will benefit El Rescate, which provides legal and social services to Central American refugees. Spotted on the spacious open-air terrace at the trendy Sunset Strip hotel: Mel Gibson, Danny Glover, Herbie Hancock, Rosanna Arquette, Rob Lowe, Bonnie Raitt, Daryl Hannah, Jackson Browne, Morgan Fairchild and others. Event co-chairman, 22-year-old film producer Dan Sladek, said the party represented the new activism among young Hollywood stars. “This is an Oscar party with a conscience,” Sladek said.