A lot of Mastodon users follow hashtags, so including relevant hashtags (#accessibility and #blind seem like good starting points) might be a good idea. Tagging groups, such as @accessibility, might also help.
As I’ve already said, for someone who is not on Mastodon, it’s pretty much worthless to try and discuss Fediverse post accessibility as applied on something that isn’t Mastodon with people who are on Mastodon. And Guppe is practically exclusively used by Mastodon users.
One example: Many Mastodon users have stuck in their heads that you can’t post more than 500 characters in the Fediverse. For even more Mastodon users, “alt-text” and “image description” are 100% mutually synonymous and mean the exact same thing. Image descriptions, no matter what they contain, always go into the alt-text. It’s like a law of physics, deviating from which is unimaginable.
If you talk about describing or explaining something in the post text body, whoosh, it flies over their heads. No matter how much sense that’d actually make.
Not to mention that you have to keep every post and every comment at 500 characters or below, otherwise a large number of Mastodon users will pretend you aren’t even there or mute or block you outright. I know that from personal experience. And there are things that simply can’t be discussed in glorified tweets.
Also, Mastodon seems to only know two kinds of pictures. One, screenshots of social media posts. The stuff that requires transcripts. Two, simple real-life photographs, especially cat pictures.
Edit: I over-emphasized the point about reaching a broader audience. If you want to discuss a narrow topic but you don’t want most ActivityPub users to see it because you don’t value their input, I guess Lemmy is as good as it gets.
Ideally, I’d discuss this topic with people from all over the Fediverse. And I want these people to discuss it with each other within the comments section. Mastodon users who really care a lot for accessibility, who want everyone’s needs to be catered to, and who are shooting for WCAG level AA, just as well as users of Pleroma, Akkoma, Misskey, Firefish, Iceshrimp, Sharkey etc. etc. who have much higher character limits in their post and users of Friendica, Hubzilla and (streams) who do not have a character limit.
I don’t just want a bunch of one-on-one discussions between myself and someone else. I want to discuss such matters with Mastodon users and non-Mastodon users, and I want the Mastodon users and the non-Mastodon users to read and reply to what the other side has written.
I want people on non-Mastodon projects to tell Mastodon users who only know Mastodon what things are like on other projects. I want Mastodon users to tell non-Mastodon users how important accessibility is and which aspects of accessibility is how important. And I want to learn from this discussion.
I want to read opinions and ideas from all over the Fediverse. And I want users from all over the Fediverse to read these opinions and ideas.
And in particular, I want to discuss with them edge-cases in accessibility that go far, far beyond Twitter/Mastodon screenshots and cat photographs.