Mark, I understand the distinction you are making - and that a reputation is a relationship kind of entity. It can only exist (in an active fashion) inside the mind of a person and it is also about some person or persons (specifically their history and their personal nature).
I can think about my character, my moral history, etc. And I can think about my reputation with others, and others can have their ideas about my character, etc., and one could abstract or summarize those ideas of others to say what my reputation IS. It is kind of wierd that I can talk about my thoughts about my reputation, another specific person's idea of my reputation, and my reputation in the world as a whole.
You said, "...the defamed person owns her history, in the sense that it is her history..." I'm not disputing the underlying concept, but want to clarify an aspect of "ownership" - to own something is to have property rights in it. Property rights are always about actions that can be taken relative to the object or entity in question without anyone's permission. For example: I own my car (purchased it outright) and therefore I have the right to sell it. If I had leased the car, I would still "own" some things about the car. I would have the right to drive that car (an action relative to that piece of property) as long as I stayed good on my lease terms.
For the points you are making, you'd need to discuss the actions that can be taken by the owner of the property in question. Only then can you say what would constitute a violation of that right.
You said, "...she does own her own life." Yes, but whenever Rand discussed the right to our life, she was careful to put it in the context of the actions that constitute life - breathing, being able to exercise one's choice, pursuing ones values, etc. I'm not seeing how one "owns" their past actions. My past actions are "Mine" but that is in the sense of who did them, not in the sense of who holds moral or legal property rights to them. That is something else and would need more said to be established.
Despite my nit-picking, I think you are on the right track. There must be property rights involved. And defamation is akin to vandalism - damaging someone's property.
The hard thing to deal with is that "reputation" is not just in the head of one other person, but possibly in heads of a great many people and held very differently from one person to another. In this sense it has a kind statistical existence. For example, what is the reputation of Ayn Rand? She isn't alive so her defamation property rights don't exist, but she does have a reputation and one that varies from person to person. That makes reputation a thing that is hard to nail down metaphysically and epistemologically.
Defamatinon is an act. It is committed by a specific person and there is a living person claiming to have been damaged by the defamation. Character assassination is just one kind of defamation. You are on the right track to say that "reputation" in the eyes of third parties is somehow where the damage is located. But perhaps rather than point to an "ownership in past acts" it would be more a loss of liberty. There is a kind of universal liberty we can speak of (Everyone has the right to not be imprisoned if they have not violated any one's rights). But we each have a degree of earned liberty - In a sense I have earned the right the good opinions of thirdparties to some minor degree as the product of my past efforts. For example, say Steve Jobs were still alive and said he was going to release a new kind of gadget. He, because of his past successes, has earned a certain level of market interest in what he says. But if someone made up a lie that his claim was false because he had started taking mind-altering drugs they would be taking away a market share that he would otherwise have earned. Kind of like when someone stops a person from travelling a specific public route to get to work, they might be causing a damage that won't happen till long after the blocking (e.g., missing an appointment by being late that turns into a lost revenue.)
I think this is what you were saying when you said that people rely on their moral history in dealing with other people being reduced by defamation and reducing their freedom. We put 'money' into our reputation bank with today's moral acts, and if someone wipes out some part of that account, there will be less there to draw on in the future. We have been vandalized.