In 1980’s I took Western Civ with Prof. Karl Weintraub (another famous professor) as a 4th year. I graduated but stayed in HP for other reasons. A good friend of mine arranged for the two of us to do an off the books reading course with him (on Jacob Burckhardt), meeting weekly. I wasn’t even a registered student at the UC at the time but that was completely irrelevant. My friend is now the President of a well known college and I went off to medical school.
So perhaps this sort of thing happened not uncommonly?