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

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