OB,
I agree with the reality of life, and we would all accept the captains choice as being good enough, and I doubt many would disagree. But I'm sure there is a perfectly good reason why the law specifically says the kicker chooses, and not anyone else, and it's probably because of exactly this.
Captain wants to kick, and kicker says he can't get it, dispute happens, either the captain goes for it, or he agrees with his kicker. Most captains if their kicker said they couldn't do it wouldn't argue with them...but most isn't all!
So, in law Phil E, yes you were wrong because the law clearly states the kicker, not the captain. Is anyone really going to complain to you about it? I doubt it.
OB, How do you know he's the kicker? as we are talking mainly about last minute kicks, you would know who had been kicking, but basically the guy standing next to you with the ball in his hand is going to be the kicker. Maybe a simple question "Are you the kicker?". If a yes, then that's good enough.