I could be mistaken but I think the Active Users Mod is already set up for that feature. If a member selects to be anonymous, the Admin should still be able to see the member. I will do some quick testing to see if that is indeed the case.
Go to Admin Options -> Config Active Users and set Anonymous Override to Administrators Only (or Moderators if you want your moderators to be able to see them).
Search is your friend “I was having a mildly paranoid day, mostly due to the fact that the mad priest lady from over the river had taken to nailing weasels to my front door again.”