Perhaps the empty chair should be called the "projection chair." We imagine someone in the chair and project that image upon the chair. The primary mechanism of transference is projection. Does that mean we in transference with a chair?
I recently finished reading John Nolte's book, The Philosophy, Theory and Methods of J.L. Moreno: the man who tried to become God. It is excellent and I can't recommend it highly enough. I believe it is the best book written about the subjects of the title.