There is an option in Display Settings called "Show a ID tags on notes?" which, if checked, will add the node ID (in [id://nnnnnn] format) to the title/author bar for each reply in a thread.

It didn't take long for hossman to discover the patch. :-) In fact, he was a bit too quick - I started working on it this afternoon and demerphq finished it up and applied it. The format will be tweaked slightly, but the functionality is there. Enjoy.