Just a thought from another oldtimer that predates even bulletin boards, but you might want to make your trim program so that it only archives people with less than, say 100 posts. Perhaps even a lower threshold like 50.
If the program dumps trace for anybody with more than that, it would give you some leeway about more prolific, and long lost posters.
Dumping that trace to a web page entitled, "missing posters" with whatever contact info you have would give the rest of us a chance to say "hey, I know that guy" and to help get them back into the fold.
The downside to distributed responsibility activities is that too few people are able to stand up and say "heck, why don't I do this...". However, many are able to respond well to requests for help along the lines of "hold this" (or in this case, "find xyz and say hello").
-CTH