The only person who has posted in the tag [accessible-categories] is me, ... just that it's too small to even register when people are looking through tags to follow.

In that case, I really think you should e-mail the people whom you think may be experts. This way they will see the tag and subscribe to it!

But in all seriousness, short of e-mailing the people and asking them to take a look, I don't think there's any way to really make sure the intended targets see a question on MO. Good luck!

A much better thing to do, since you seem to have some knowledge who these set theorists/logicians/model theorists are, is to send them an e-mail with a link to your question on MO.

Yes, this is the other option, but it seems a bit silly to even bother asking them on MO.

If the users do not subscribe to a given feed, it is because they made a choice.

The only person who has posted in the tag [accessible-categories] is me, and I don't think that people will bother subscribing to such a niche RSS feed. I don't think that there was any conscious choice to not subscribe to that feed, just that it's too small to even register when people are looking through tags to follow.

A much better thing to do, since you seem to have some knowledge who these set theorists/logicians/model theorists are, is to send them an e-mail with a link to your question on MO.

For example, the set theorists, logicians, and model-theorists here all seem to know a lot about accessible categories (I assume because they have some sort of application to model theory, according to the title of Makkai-Pare). When I ask a technical question about accessible categories, would it be wrong to tag them [lo.logic], [model-theory], and maybe [set-theory], even though it's not directly about those subjects? tagging it just as [ct.category-theory] and [accessible-categories] makes it so the people who can answer it easily probably won't even see it because they're not subscribed to the ct.category-theory RSS feed (or whatever technology people are using these days).