Lol, honestly I'm already deeper down this rabbit hole than I ever thought I'd be.
So far as I can tell, Kbin doesn't provide a way to specify alt text on embedded images at all. You can attach an image to the post using the GUI, and specify alt text that way, but it will always be at the end of the post, and you can only have 1. Not ideal.
Re: Syntax support. There's no official spec for markdown, but there is a canonical syntax guide which describes the format I'm suggesting. You're correct that it's not very widely supported in implementation though.
I think I created confusion on that ticket by talking about the codeberg.org implementation, which does support the suggested syntax, by populating the title attribute. I wasn't necessarily suggesting it as a solution, and it sounds like that's not a great implementation anyway.
I created even more confusion by talking about a plugin that sighted devs or testers could use to see alt text, and gave a few examples of elements to look at. None of that is really relevant.
I did try to clarify as a comment on the parent post here if you're interested.