So it appears to me to be the logging is not recorded when done as a function of the owner/moderator privileges, but is done when changes are done through the individual's subscription tab.
I have checked back to the point in time when I and another moderator of a group chaned our display names "via the web". Both changes appear in the Activity Log.
J, you mentioned a DN change being logged when the changes are done via email, but I can find no email command available to change the display name? Can you point me to it?
I have seen that "via email" in one or two Activity Log entries as well. and would also be interested in knowing details of the process because I cannot see "how" either.