Egghead and King Tut appeared in a few episodes of Brave and the Bold as well. i think they were just background characters though.
There were cameos by a bunch of '66 series villains in "Mayhem of the Music Meister" (I think it was), but they weren't identified by name because of the legal issue. I think that a version
of King Tut showed up in something, but it wasn't the same character who appeared in the '66 show; after all, King Tut is the nickname of a real historical figure, so it's not a name that can be copyrighted.