What do you mean by "participation" and "filtering"?

If you mean a post, I don't know of anything in Listserv, but if you use "changelog", you might take snapshots of the subscription list.  Write a script and run it whenever you have a snapshot 6 months old. The script would ... for all subscriptions that exist in both the 6 month and current snapshots, find those without a post in the change logs for that period. Those are your lurkers.

Cheers, Wayne

To unsubscribe from the LSTSRV-L list, click the following link:
http://peach.ease.lsoft.com/scripts/wa-PEACH.exe?SUBED1=LSTSRV-L&A=1