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 really don't think so, unless it's part of someone's explicit wishes/religion etc. It's important tor respect what they want rather than assume. Otherwise I view dead bodies the same as poop. They need to be disposed of properly and all that. Also who's to say a traditional burial is ethical? I'd feel claustrophobic.
I think when it comes to consent, we usually do assume that someone's not okay with something (or err on the side of caution that they might not be) rather than assume it's okay to do something to a person unless they've explicitly requested that it not happen. It works the other way round, where we only do something to them if they've said it's okay. Of course there are exceptions to this, such as helping someone when they can't help themselves if it's extremely critical or if it doesn't violate them at all (like putting a warm garment over someone who's fallen asleep in the cold), or what might be argued as necessary to do for a child's development so that they can live a functional/healthy life. And then there are cases where it'd be ideal if we could not do anything, but the situation forces us to choose an option of what to do, such as dealing with someone's dead body. In those cases I think the safest thing to do would be to choose one of the most common methods of interment, since 1. The person was likely aware (though not necessarily) of the main methods of disposing of someone's body that are usually practiced by humans when someone dies, and probably had the opportunity during their life to object to them and request something different if they didn't like it. 2. Those methods are generally regarded as the most respectful options available, and so statistically someone would be likely to also agree with that sentiment. 3. They're also arguably some of the least invasive/violent/brutal ways of dealing with someone's body, though of course none of them are completely nice since you're disposing of a dead body after all.