I know this is anticipating technology that wasn't around
Actually, advances in science have made Pike's condition more plausible for the 2010s audience, not less...
Back in the 1960s, few in the audience would have been aware of the wide range of brain damage types and cognitive disorders. Even the writers would have been thinking in terms of "His mind is locked inside a damaged body we can't repair". But from the viewpoint half a century later, it's pretty simple to assume the opposite: "His damaged mind we can't repair is locked inside a body we can repair (but to what end when he can't use it anyway?)".
That is, Pike would not suffer from lack of vocal cords. Nor would he suffer from being a total vegetable. But beyond these 1960s alternatives, there's the plausible one where Pike has lost the ability to use language. Yes, he can be positive about things, or negative. But no, he can't form a coherent phrase to describe these feelings, or even an incoherent one. His chair can guess on his positiveness or negativeness and relay that to the outside world, but his thoughts will not be formulated into words, or even into what exists before a word is formulated and relayed to the assorted speech organs.
Not even a mind meld would necessarily help, then, because Pike isn't merely unable to say out loud "I want that glass of water", he's unable to
think it out loud, either. That's still completely within McCoy's parameters of "His mind is as active as yours and mine, but it's trapped inside a useless vegetating body". McCoy would simply equate the body's (the brain's) inability to provide language with the body's inability to provide motion, or heartbeat, or digestion. The mechanistic trouble with turning thought into language would not mean an enfeebled mind, it would merely be another variant of being physically crippled.
There'd be no point in having a voice synthesizer in the chair in that case. All it could do would be to put "yes" and "no" into more complex sentences, arbitrarily chosen and not reflecting the thoughts of Pike any better than simple "beep" and "beep-beep".
What the chair could use, I guess, is a pair of manipulators. Pike is apparently capable of controlling his movements (and we can assume the chair is capable of cross-country movement, despite its Dalek-like design), so he might derive satisfaction from being able to manipulate things, too. But perhaps such aesthetic-psychotherapeutic improvements would come later? It's been "months" since the accident, but perhaps a few more months would see Pike given a proper android body?
Timo Saloniemi