Okay Think I may have found a post with some code that may help to achieve this with some modification. Maybe converting it to a bbcode wrap?
Theme component Right side Blocks also has some code for recent replies may also work.
Basis to modify to a bbcode wrap that will maybe use topic I’d as a variable to fetch topic in box to display in a post.
Have it’s own header at top of box with link Add comment. And have first post display none.
Other potential application beyond adding a comment window. If doing a single post. One could also say use it in an announcement only category to display a pill or other such things.