A healthcare union trying to organize workers at Boston-based Beth Israel Deaconess Medical Center has assailed the hospital's physicians group for charging a late-night fee for patients who come to the emergency room after 10 p.m. Local 1199 of the Service Employees International Union drew on Medicare claims data that show patients treated by the Harvard Medical Faculty Physicians at five hospitals in the state were billed a surcharge of $30 if they were seen between 10 p.m. and 8 a.m. The doctors group and Beth Israel representatives said the fee was a common practice in Massachusetts and nationwide.