What about an opt-in bot that uses the Mastodon app API.
If someone replies to one of your posts with #alt4you to provide alt text for an image you posted (whether it was missing or just poor) and you reply to their reply saying #altAccept or something... the bot revises the original post and applies the alt text.
So your original post can attract helpers with #alt4me and helpers use #alt4you in reply and you #altAccept in reply and bot makes it so.
Needs read & edit access.
Edited 13d ago