Points 3 & 4 could be combined: like .plan, each user has an o+r .status file. Its contents are a user's status message, but its modified time is used specially to indicate when that user was last active. Anytime the usuc client writes a message to a channel, it would simply touch the .status file. And whenever an expanded usuc or some other tool lists the users in a channel (that is, all users in the group that owns the channel file), it would mark as idle any who had a .status mtime older than 30 minutes, say.