Curious if this question was ever answer?
To my knowledge there is not a way to do this. You either have to show username or First/Last name. The most anonymous you can get is to hide full name by default and from user mentions coupled with making name visibility private to the user. That will only show username to both guests and logged in users. Site admins still see the first/last name though. The settings for this are in Admin Console > People > Settings > Global Profile Settings.