I have now done a bit of searching online that I probably should have done before asking that question. Sorry about that.
[Apparently][1] in the current version of Discourse you can take a link to a post and swap the `/t//` portion of the URL for `/raw/` and get the original source for that post.
e.g. `https://community.openstreetmap.org/t/link-in-imported-old-forum-post-has-no-target/145439/7`
becomes
https://community.openstreetmap.org/raw/145439/7
I realise this is now totally off topic, but this thread is also the top search result for "Markdown source" so I thought I'd put it here for future reference.
[1]: https://talk.tiddlywiki.org/t/discourse-markdown-output/8344