Outside of James Gregory, William Windom and Corey, I am hard pressed to remember another TOS actor who appeared on Barney.
Do you count the TOS movies? George Murdock ("God" from ST V, as well as Admiral Hanson in TNG) had a recurring role as Lt. Scanlon of Internal Affairs.
Looking over IMDb's cast list for the series, I also find Vic Tayback, Ed McCready Jay Robinson, Meg Wyllie, Janet MacLachlan, Jon Lormer, Ian Wolfe, Michael Pataki, and Victor Brandt. And if we count the movies, there's Alex Henteloff, Christopher Lloyd, Roger Aaron Brown, and Allan Miller.
And it has gone the other way a couple of times. Gregory Sierra was in DS9's "Second Skin" and Ron Glass was in VGR's "Nightingale." Guest stars who did both Barney
and modern Trek include David L. Lander, Kenneth Tigar, Kenneth Mars, Stefan Gierasch, K Callan, James Cromwell, Robert Costanzo, Nehemiah Persoff, Richard Libertini, Barry Gordon, Stanley Kamel, Joanna Miles, Tricia O'Neil, Warren Munson, Alan Oppenheimer, and Earl Boen.