As for the question of cameos mucking up the works, I read about this Bob Hope movie called "The Paleface," a western comedy. Apparently, according to the IMDB trivia (or maybe Wikipedia?), Bob Hope had so much influence back then that he got a crazy amount of cameos from all the western superluminaries of the time. He had James Arness as Matt Dillon, James Garner as Brett Maverick, Fess Parker as Davy Crockett, and others. It seems that no TV presentation or video release of The Paleface has ever been totally complete, because they weren't allowed to use some of the cameos outside of the theatrical release. I don't know any of the whys and wherefores of that, but there might be a similar situation like that for Batman.
In that case, the issue would probably be the use of copyrighted characters, not the appearance of the actors. Although it's true that a couple of the "window cameos" in Batman
were performers in character as their contemporary sitcom roles (though not explicitly named as such), including Werner Klemperer as Col. Klink and Ted Cassidy as Lurch.