Is there any source which specifically discourages or prohibits drawing the face of Avraham, Moshe, or any other biblical/rabbinic figure (besides for any issue of drawing people in general)?
Picture books published by religious organizations meant for young children often leave out the faces of these specific people, and I vaguely remember hearing about a midrash or something which discourages drawing the face of Avraham.
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.
in order to make sure nobody thinks that such great people could actually be captured by such mundane images Actually even great people are still human and show up in pictures. This seems to equate them with divine beings c"v...