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?
Being literally dead, as in no brain or heart activity, is a pretty good place to draw a line.. totally fixes your slippery slope.