C. C. Beck is the beloved artist who brought Captain Marvel to life in Cap's golden-age comics. I found a few instances where Beck made some guest appearances in Cap's comics! Here are the ones I have found so far.
The first appearance I found was when I was researching the origin of Captain Marvel Jr and ran across a story in Master Comics #22. In this story, Sir Clarence Beck has become a victim of Dr. Eternity - partner in crime with Captain Nazi! (Peter Costo in the picture below was probably Becks assistant, Peter Costanza. Costanza also made a few cameo appearances as I mentioned in my article here.)
Beck next appears in Captain Marvel Adventures #18. In this appearance, C. C. asks a question in the The Mental Marvel Quiz. His question is, "Who is Captain Marvel?" The answer comes from Freddy Freeman (Captain Marvel, Jr) who says Captain Marvel is "the world's mightiest mortal!" I think it's pretty funny how Billy and Freddy respond to C. C.'s question!
The final appearance I've found is in Captain Marvel Adventures #97. I'm basing this information from a notation on that comic in Comic Book Plus that states it's a cameo appearance by Beck. It's a great appearance as the story is about Captain Marvel saving the world from an evil artist with a magic eraser that can destroy the world! Please note that the features of the artist in the picture below are very similar to the Beck in Master Comics #22 listed above!!
Beck made an appearance in the '70s Captain Marvel comic series in Shazam! #4 which I wrote about here. And C. C. Beck made a guest appearance in Shazam! Convergence as drawn by Doc Shaner!
These are the only golden-age comic appearances that I'm aware of, but be sure I will update this article as I find more.