According to Jeffrey A. Rydberg-Cox's, Overview of Greek Syntax:
Active Voice: The active voice denotes that the subject is the agent of an action.
Middle Voice: The middle voice denotes that the subject is both an agent of an action and somehow concerned with the action.
Passive Voice: The passive voice denotes that the subject is acted upon.
With that said, your comment that "No emphasis is placed on the character of the person visited; instead, the emphasis is on the disposition of the visitor." makes no sense. Mary is the subject acted upon by grace, not the Angel. In other words, the word kecharitomene is completely about Mary. She is the subject of the verb, not the Angel.
Pray for John Paul II
I know you won't respond (as apparently my posts in regards to Greek do not exist), but you're only half right here. She is the subject of the verb, but as it is in the middle/passive (there is only one way to denote the two voices in the perfect participle), she is being acted upon. When I say, "I was hit" it doesn't say a whole lot about me except what happened to me. It doesn't say that I have a quality of "hitness" which makes people hit me. That is the stretch which you are attempting to make when you say that the participle in the middle/passive shows us something about Mary. She is the object of grace (a far better translation of charitoo than favor, synonomous to the Hebrew word that is translated in KJV "favor" as in "Noah found favor in the eyes of the Lord"). It's not that she has some "graceness" which causes her to be the object of grace.