I'd thought similar myself, but I couldn't get it to sound right in my mind - i.e. it seems "too forced".
Perhaps something more related to "sequence" or "first shot" or something. A character with high charisma should seem friendly, on your side, harmless etc - regardless of whether they are actually friendly, you'd be tricked into believing it. One with low charisma should seem "obviously evil" from the moment you spot them. In terms of game mechanics, I suppose you'd be able to shoot a low CH character immediately, whereas a higher CH would be "protected" for a few seconds (or turns?), or until they shoot you first. Perhaps it would affect the chance to hit, as you suggested, by means of being uncertain about what you were doing?
CH10 should be the most awesome, friendly, seems like the nicest person in the world, you'd never want to shoot them - you'd want to make friends. A charismatic bad guy could be stood next to two dead traders, with a gun pointing at them, and persuade you that "it was like that when they got there", and you'd believe it. CH1 should be the hated local rapist, paedophile or child killer - to be killed on sight, and assumed guilty of all possible crimes. Anything in between that is on a scale between the two.
Still seems a bit "forced" though. A slightly less forced option may be the ability to get extra combat training / extra perks from specialist NPCs, who would only teach the better skills to "nicer" characters. Something's available to everyone, but the higher the CH, the higher the bonus i.e. they like you more, so spend much more time training you. That might actually make it useful, without having too great an overhaul of the existing system.