An interesting idea, but the problem that strikes me is this: changing "the slave owned by John" to "the person enslaved by John" seems to imply that it was John, specifically, who forced them from a state of freedom into a state of enslavement.

Which will, of course, often not be the case.

This is not a point about slavery or our attitudes to it; it's a point about changing unambiguous language to potentially ambiguous language...