I am very new to pathfinder (in my first campaign still), but I do have experience roleplaying a stoic yet still uncharismatic character on a MU* to give my thoughts on the subject.
He was the kind that is brooding, self-centric, and often avoids other people and would very much represent a below average charisma score. You would think that this kind of person would get all of the chicks, but in reality it only makes people not want to roleplay with you because often social encounters are reduced down to saying nothing, or rejecting people to continue being by oneself. I eventually had to open up my character more. The kind of character that rejects the world around him to live in his own thoughts can very much represent a low charisma.
Basically he was more of an Edward Cullen than the kind of strong, silent type that speaks with action and integrity and has a powerful presence to him.
Since from what I understand, participation is a valued thing in Pathfinder, I wouldn't recommend allowing such a character because it would only leave the person idling most of the time until it is time to roll the dice.