Jonah,
I don’t know if there are any examples prior to the Geico Cavemen of characters from advertisements transforming into TV show characters, but the practice was not entirely unheard of in broadcast history. Back in the 1930s, when networks and admen worked together to create the first serial dramas, there was a porous separation at best between commercial characters and program ones. One of the original dramas, Today’s Children, came to set much of its action in the kitchen of the Irish-American matron. It just so happened that Pillsbury had become the sponsor, replacing the original sponsorship by two General Mills laundry products (La France Bluing Flakes and Satina starch, thus the origin of the term “soap opera”). The plot lines often involved baking (sounds mesmerizing, I know) and involved references to a character named “Mrs. Pillsbury.” But as far as I know, the character never became a voiced one on the show.
However, in a later General Mills-sponsored series, Betty and Bob, the trademarked character Betty Crocker did make guest appearances. Somehow, she always seemed to culinary solutions to whatever was ailing the main characters.