If we consider post-mortem rights to matter morally, then something like necrophilia or defiling someone's body after their death would be immoral even if they don't experience it (obviously) and even if they don't have any family or loved ones around to witness it or know that it happened. As an extension of themself, their dead body has intrinsic moral value as far as an obligation to treat it respectfully in accordance with what the person would have wanted or been okay with, not merely instrumental value that it serves to loved ones or the environment. And since we consider a person to have more moral value than the environment (otherwise it could be ethical to kill people to remove their environmental impact), even if it was more harmful to the environment to dispose of a body in a certain way (e.g. standard burial or cremation) over other methods, it would then still be ethical to dispose of it in a way the person either opted for or was likely aware would be done, rather than a less commonly known/practised and more invasive yet eco friendly method such as sky burial (putting their body on a mountain top and letting vultures tear it apart).
In other words someone's bodily autonomy extends into death because they lived in their body their whole life, have a personal attachment to it as part of their identity, and just as they likely wouldn't want it violated while alive (even if they were asleep for example), also likely wouldn't want their body used for something disrespectful or really anything other than a standard form of interment (process of disposing of a body or putting it in a final resting place) that they would probably be aware would happen when they died, or is as generally uninvasive/dignified as possible, unless they specifically consented to something different or made a particular request for what would happen to their body.
IF all of the above is considered true, then (or just in general) wouldn't it be unethical/disrespectful or a rights violation to preserve a human's shrunken head for hundreds of years and then have it in an oddity collector's shop to sell it to people to display in their houses?
I would also add that it seems that rights are a human concept/social construct, even just in the sense that we're interpreting what we believe to be ethical/right/moral, even if it's objectively correct; or we're enforcing laws based on what people believe is correct, or in some cases what serves certain people personally at the expense of what most people believe is right if the laws are corrupt/undemocratic.
So I think if we're going to claim that a certain right "just is", since we're the ones creating these concepts even if it's based on our observation of the world and an interpretation that was theoretically objectively correct if not a belief, it falls on us to rationalise and describe how we're coming to these conclusions and what we're basing this assertion of a certain right on. Otherwise, "it's a human right because it's a human right" is just circular reasoning and has no explanation. How are we formulating our basis for what is a human right? Is it legality? Is it moral beliefs or what we reason (or even logically prove somehow) is objectively morally right? Or ... what?
For example, in the case of animal rights theory, many people believe that there are moral rights that animals hold as moral patients, i.e. "negative rights" (= freedom from something being done to an individual) not to be exploited and killed by humans (moral agents), which extend logically from the belief (or fact) of human rights also being morally correct. And in this view, humans by way of our laws, do hold legally the "positive rights" (= freedom of an individual to do something) to exploit and kill animals, but these legal rights are simultaneously violating the moral rights of the animals to not have these things done to them by humans/moral agents.
In this case too, similar to what you said about the human condition, we could argue that something about the condition of animals (which could for example be sentience/consciousness, which they share with humans who are also animals), is the basis for them having these rights, but even then we're still speculating based on what we believe is either subjectively or objectively moral (since in that case obviously what's legal is in contradiction with what's deemed to be moral), and I'm not sure what third definition of rights could be being applied there whether it be in the context of human rights or animal rights.