> "
How do you reason that people have the (presumably absolute) right to alter than bodies however they please? How do you justify that claim?"
If you couldn't gouge out your right eye and cast it from thee (Matthew 5:29), would you still have freedom of religion?
> "So you cannot say you have the right to self-harm. Now, if you have, say, a defect in your body that is preventing the proper functioning of your body, then we may correct this defect through intervention. This is restorative, that is, it seeks to restore the function due to the body by virtue of how it ought to function. This is the basis of medicine."
What about vaccination and such where the intent is to cause a small amount of harm now, to prevent a problem you don't have - and may have never faced in any future. This is not restoring a defect, it's augmentative and preventative. Do you not have the right to cause that small harm? And yet that's in opposition to your "direct self-harm is even worse morally than harm of others because it is in direct opposition to one's own good". It's in alignment with one's own good. So, for example, are the torn muscle fibres when weight lifting, or the buildup of lactic acid causing muscle burn when running, or working through the night with no sleep in an emergency.
Less seriously, piercings for jewellery are self-harm and for one's greater happiness in appearance. Do people have no right to wear small pointy uncomfortable high-heel shoes? Is that immoral?
> "I think it's pretty obvious that right implies morally acceptability."
I think it's pretty obvious that there isn't a single "moral acceptability" which everyone agrees on. For example some have the right to eat animals and others consider that morally inacceptable. Some US states have different rules on public nudity than others, and some groups of people have different opinions on the morality of it.
> "You can't claim to have a right to something evil"
Not sure I follow this one; you can't really have a right to murder others, but in the sense of self-harm is it really "evil" to hurt yourself? What is evil and also doesn't involve harm to anyone else?
> "For example, do you have the right to lobotomize a perfectly healthy brain?"
One could argue whether a "perfectly healthy" brain would choose to lobotomize itself, but let's say it does - why do you say that's "very problematic"? The main objection I have - assuming the person is considered sane and healthy - is who is going to pay for the care afterwards, i.e. the harm rolls over onto others and is not purely self-harm. If you could sign that away - it's basically a slow and horrible suicide method.