And the fact that there's no known good examples of slaves being happy with such an arrangement is a very strong argument in favour of any sort of right to own slaves being a unlikely sort of right that a society would ever successfully and sustainably adopt. I just wouldn't rule it out on principle - if there's anything like a "universal truth" I would accept it's that other societies/cultures need to determine their own rights for themselves, and they can't be imposed.
I'm not sure the union and the confederacy saw themselves as separate cultures though? And either way, I'm not convinced that it was an acceptable use of force (granted, it's not a subject I have any great depth of knowledge in). There are parts of the world today where certain members of society live in conditions not far from slavery, and while I very much hope those societies can in time see the advantages of agreeing on and adopting a more free and equal set of human rights, I don't believe it's justified to use force to impose them just because we're so certain of their "unalienable" nature.
The Janissary elite drawn from the devşirme system of child levy would be the obvious example, particularly toward the end when they controlled many of the state assets and staged palace coups to get the sultan they wanted.