Leonard Bernstein was an outstanding pianist who wrote some of his best music for the instrument. Before he died last year, he entrusted his recent piano writings to Michael Barrett, who apprenticed with the maestro for almost a decade. The keeper of the flame has the characteristic Bernstein versatility: he's a recitalist, conductor, and man of the theater. Trained at San Francisco Conservatory, Barrett, still in his 30s, has already won a Grammy and is in charge of musical chores at the Lincoln Center Theater--on top of a hectic itinerary on the road. At this Dame Myra Hess concert, he's set to give Chicago premieres of two major works from the Bernstein archive: Meditations Before a Wedding (1984) and Thirteen Anniversaries (1989). The latter is a series of parting valentines to friends, colleagues, and family from a composer who loved extravagantly. Schumann's Papillons completes the technically daunting and emotionally charged program. Wednesday, 12:15 PM, Preston Bradley Hall, Chicago Cultural Center, 78 E. Washington; 744-6630 or 346-3278.