In reply to J.C.Ray.

Yes, Sall's paper addresses this on p. 331. Short answer: the partial leverage plots make sense. He describes how to interpret a partial leverage plot if the sole regressor is a classification variable. Also the case of a classification effect and a continuous covariate.

I don't think quadratic terms or interaction effects present any problems. They are merely columns in the design matrix and are treated the same as the main effects.