Heureka!
Ok, I didn't find "it", but I found "something"!
Do a search for a keyword in discussions : then look at the entries for the discussions: They report "Last entry by USERNAME at TIME"...
So the thing we want is in there somewhere... now we just have to tweak it out into the open! :D
I put the request in the suggestions box... I can't think of a way to hack this, since, sadly the search engine as-is doesn't seem to look at the names of participants, nor at the text posted.