(China, by the way, also had the same understanding -- although the state was stronger in China, and so was more often able to defy the natural law and get away with it. Look at the long history of Chinese censors, who risked torture and death to remind the emperors of their duties.)
And as for "laws that are so obvious they need not be stated anywhere," what's common law if not that?
Rights are what you assert for yourself. You can, of course, assert that you have dominion over others, but you'll have to persuade those others to agree with your if you hope to exercise that, which is unlikely but not impossible - churches are full of people who want to be relieved of the burden of freedom qua moral decision-making, and are thus fertile recruiting grounds.
Of course you may also assert rights but find them denied by law for one reason or another. In which case you can choose to give up your rights or fight for them at risk to your person. Laws are merely the product of political process, which may or may not be corrupt. If you feel a law is unjust, you're off to a good start because ultimately law is merely the servant of justice, and if you wish to rebel against an unjust law or practice you'll often find many supporters.
Rights have a significant moral dimension as well as a legal one, so just because someone denies the existence of your rights doesn't mean you have to obey them. You can defy or subvert their authority, and if your intentions are pure then you should. By pure I mean not being a hypocrite; the broadening of your individual rights is not predicated on the shrinkage of someone else's.
On the other hand, if you steal from someone and they arrest you, then that person had the right to be free from your theft of their property.
You can assert whatever rights you want, but they aren't rights until other people agree that you have them and collectively decide that no attempt at punishment will follow you exercising them.
For context, I also hold the position that while being incarcerated obviously limits your physical liberty and maybe others, true liberty is a a state of mind; if you both take responsibility for acting on your authority and consider your incarceration unjust, your psychic liberty is intact and can't be mechanically confiscated from you. This might seem like a meaningless distinction, but the consciousness of one's fundamental freedom is what sustains many political prisoners, and conversely what authoritarians most fear and work to suppress.
I have to say I'm astonished by HN lately. I havae not seen so many badly rationalized apoloegtic arguments on slavery in my long life nor in 10 years at HN as these past three months.
In my mind, I'm making a distinction for a very real reason -- to say that, e.g., freedom of speech is a universal human right explicitly whitewashes over the fact that a tremendous number of people are deprived of that right.
What I want to make very explicit in the language I use is the idea that you have only the rights that you can reasonably pursue your daily life comfortably exercising. I find slavery abhorrent, as does pretty much everyone. Which is why I don't want to pretend like the problem just doesn't exist anymore, because after all, everyone unambiguously has the right to an existence free from slavery. They absolutely don't. There are plenty of people who can point to that idea as a travesty.
There's no specific number of people they need to convince otherwise to gain that right. They need to exist in a governmental, societal, political system that demands that that right be conferred on them. Until that happens, they don't have it and we shouldn't be saying otherwise.
Distinction you posit is in fact between a priviledge and a right. Priviledges are bestowed by a form of governance, rights exist regardless.
If we consider society in terms of game theory, law provides the rules, but the object of the game is to rewrite the rules through the exercise of power, for which you must employ strategy. The assertion of moral rights is a strategic objective.
"And as a matter of constitutional law, we strictly prohibit doing that in our own legal systems."
If you are talking about the USA, this is fairly common, at least in so far as civil cases. Any time the Supreme Court recognizes a new right, it opens the door to new lawsuits based on those rights, which can be retroactively applied. Consider the 1960s and 1970s and 1980s, when the Court expanded rights at a rapid rate. Thousands and thousands of lawsuits were retroactively filed based on those new rights. People who had been victims of racial and sexual discrimination sued to gain justice for decades of abuse. Once the Court recognizes a new right, the assumption is that the right had always been there, since the beginning of time.
It really wasn't. We would like it to have been, but at the risk of stating the obvious, it's not an inalienable, fundamental right if someone can take it from you without fear of repercussion. Slavery was legal -- the scare quotes add nothing. The difference between legal and "legal" here is just personal ethics.
Now, we can of course decide to proceed today as though the laws were never legitimate, and offer reparations, for example. Or in the case of the Nazis, as you mention, to try them for crimes committed in the past. It's worth noting though that the Nuremberg trials were very controversial for exactly that reason -- many people sympathetic with the desire to punish the Nazis were extremely uncomfortable with the idea of trying people for violating international laws that didn't exist at the time of the offenses (or only existed in treaties the Germans never signed). We did it anyway because, well, they deserved it, but it's far from the truth to say that it was a routine application of some tradition in which there are some laws that are so fundamental they need not be stated anywhere. And as a matter of constitutional law, we strictly prohibit doing that in our own legal systems.