It's great to hear from you, Michael! I tried my best to check, but I could not find any other definitive reference that Rashi makes to his father's opinion. As you might know, some claim that Rashi's comment in the beginning of Bereishis ("Amar Rebbi Yitzchak...") is citing -- or at least was made in deference to - Rashi's father, whose name was indeed Rebbi Yitzchak (see the comments of Rabeinu Chayim Paltiel and also the Divrei David). But others assume this is not so, and maintain that Rashi was merely citing the Rebbi Yitzchak who is mentioned in the Midrash itself. See Naftali Seva Ratzon and also Shem ha'Gedolim.

Warmly,

Yishai Rasowsky