In Star Trek "The Menagerie", we see Captain Christopher Pike as a dang-near-everything-plegic. He has been placed in a powered wheelchair/life support system and his only physical abilities are that he can move the wheelchair a little bit, and make a light blink. By blinking the light once or twice he can answer yes or no, if he has the patience to wait for someone to blunder into a question he wants to answer.
Why didn't they teach him to blink the light in Morse Code? Scott, Spock and Kirk all knew it in "The Final Frontier" so it must have still been around. Even if it wasn't still in common use, it was written down someplace and they could dust it off for him. Heck, it was probably some sort of elective at Starfleet Academy, or at least used clandestinely by plebes after lights out.