In the case of actual slaves living in such a time, they still appealed to society both directly and through government (another abstraction,) because not even authoritarian societies are absolutely uniform, and societal constructs are mutable.

Unless you want to invoke divine will as that "standard beyond society," humanity and its arbitrary social constructs are all you have available to deal with, and some problems can't be reduced to objective and absolute terms as a result.

You could invoke "nature" but nature doesn't recognize rights. No creature has a natural right to food, safety or property. Nature engages in warfare, cruelty, violence, rape and slavery with abandon, and without even the pretext of morality. Even anarchists wouldn't want to live in a world governed entirely by natural law.