According to Peter Marshall's book, Charles Nelson Reilly did appear on Hollywood Squares. Not Brett though.
Gene Rayburn has guest appearances on more than a dozen game show to his credit, including Hollywood Squares.
In addition to PDQ, other 60's game show appearances for Jo Anne Worley include The Dating Game, Personality, Snap Judgment, The Match Game, Funny You Should Ask, Name Droppers and, logically enough, Letters to Laugh-In.
Anita Gillette became a game show staple because the Goodson camp was doing in the early seventies what they'd been doing pretty much from the beginning: Populating their panels with bright, attractive folks who were appearing on the New York stage. In many cases their stage careers never took off, and in a few cases their fame grew far beyond the bounds of "game show panelist" but people like Gillette, Elaine Joyce, Alan Alda, Larry Blyden, Bert Convy, Jack Cassidy and many more were "discovered" as game show personalities simply because they were "treading the boards" in the early seventies and were available for a little extra work.
On his site, Steve Beverly has a list of a small handfull of celebrities who make up the "Six Decade Club", covering every decade from the 50s to this one. By comparison, the qualifications (5 shows, 3 decades) made by our OP are hardly a big deal. There are probably a couple hundred or more that would qualify.