Los Angeles performance artist Lin Osterhage, at age 43, is compiling her autobiography, and even if you've never met her, she'd like to hear from you. Weary of her real past--from a wife in a rambling Connecticut farmhouse to an artist in a downtown loft--Osterhage is soliciting sentimental flotsam from other people's pasts--letters, snapshots, birth announcements and wedding invitations--to adopt as her own. In neon-pink flyers, she asks for "places I might have lived . . . friends I might have known, parties I might have attended, art work I might have done." One woman sent her some valentines from a cherished fourth-grade romance. "I'm going to weave it together, and I'll never tell which is fiction. I think I'll really accept it as my real own true autobiography." Well, that's lives.