The audience at Friday's Mark Morris Dance Group concert at the East County Performing Arts Center got even more than they bargained for. Morris made an unscheduled solo appearance in a "world premiere"--a brief but thoroughly delightful bit of improvisational dance, performed with a metal pizza pan as an all-purpose prop. Why did the guru decide to include this impromptu dance on an already generous program? "Because I wanted to do another piece," he said matter-of-factly after the performance. These impromptu solos have become a recent Morris trademark.