Hi Simon,
I was actually trying to add a button to the topic pages, to be specific, .title-wrapper under #topic-title.
Thankfully, I have since found a work-around to this in another thread on meta.discourse. This approach allows the editing of specific posts in topics via the api method ‘addPostTransformCallback’. I’ve shared it below to help any others who face a similar issue.
A drawback of this though, is that it only works if u navigate to the topic page from another page within the forum. If I were to refresh the page, the edit (in my case, the button) does not show up. I am currently trying to solve that, and will share if I find a solution!