I found a reference to this practice--of not drawing the faces of the forefathers, specifically--mentioned here, Mimidbar Matanah by Rabbi Yitzchak Pinchas Goldwasser. He writes that we avoid drawing them in order to make sure nobody thinks that such great people could actually be captured by such mundane images.
However, I also found reference to the exact opposite: that it would be better to draw Avraham than anyone else. Rav Chaim Palagi (Lev Chaim 2:171) writes that although a protruding image of a person must normally be defaced in some way in order to be permissible, when it comes to an image of Avraham Avinu, this is not necessary, since Avraham is a symbol of monotheism and idol smashing, and so no one would suspect that the image is being used for avodah zarah purposes. (He was asked this by Count Abraham Camondo, who was given an engraving of Avraham Avinu by Moses Montefiore). This teshuvah of Rav Chaim Palagi is quoted approvingly by the Tzitz Eliezer, 9:44.