In fact, let me research that -- other acting jobs by TOS main cast during TOS's run.
Shatner: Gunsmoke episode "Quaker Girl"; movie White Comanche; CBS Playhouse episode "Shadow Game"; TV movie The Skirts of Happy Chance.
Wow, as the series lead in every episode and in most scenes, when did he have the time? I can see squeezing ion movies during the hiatus, but other TV series appearances would have been shooting at the same time, most likely. When it came to hard work, there was no faulting this guy....