I do genuinely appreciate that, but that's the point: the graphical clients that do visual diffing and merging like the ones I listed all work with a bare repo

I do genuinely appreciate that, but that's the point: the graphical clients that do visual diffing and merging like the ones I listed all work with a bare repo as the remote. Heck I think even the Github Desktop application also works with non-Github repositories, and they would be the only ones that would have any interest in locking people in. Unless you mean specifically the UX of having a URL you can copy to a specific line of a specific commit in a repository, which indeed is not possible without a standard URI scheme (which does not exist) or a web client.