To answer your question, I think that wades into tricky waters because it takes some trust to add alttext to someone else's images. It would likely seem as if the first poster made the alttext when it was actually added by someone else.
I would be a little surprised if there was a good way to do it now.
I COULD see a feature added to a client like Mastodon where a person could propose alttext, and the original poster could accept it, automatically putting out an edited post.
Sounds like a good idea to me!
And purely to opine:
Adding #Alttext sounds like a pretty good use for #QT on #Mastodon, since there's been so much talk about that lately.
@volkris Very good points and interesting idea! The risk of someone applying Alt Text maliciously/misleadingly hadn’t occurred to me either (perhaps naively).