A common use case from fediverse users is to be linked out to a site, and attempt to "bring it in" to their local instance/app of choice. This is done by taking the browser URL and pasting it into their site/app's search bar, or equivalent.
For context, last night I discovered that Ghost's latest blog post didn't make it into NodeBB, due to a bug on my end. I attempted to resolve it via URL but there was no AP resource at that URL. I ended up having to query the instance actor (which I happened to already know), and looking at the outbox.
To my knowledge there is no way to find a Discourse post or topic's AP resource ID without having a local Discourse account.
Would it be possible for you to send back an HTTP 301 Moved Permanently (or similar) if the Accepts header contains one of the AP-related types?