If you’re looking for a particular poster you can @ them in the search, but if you’re trying to get an overview of which posters may have posted just once or twice there appears to be no way, as the who posted cuts off after 24.
I’m digging into it a little and coming up with nothing so far.
Here is a full list of search options:
You can try this bookmarklet (AI-coded). Just drag it into your browser’s bookmarks bar and then click it inside the thread you’re interested about. It will load all the pages of that thread in a special lightweight format (JSON) and then show you all the participants and their posts.
Video of it in action: