I like the idea of not knowing somebodies name or maybe even their level until you have introduced yourself to them. However I agree that it is essential to be able to contact a player again otherwise the whole social structure of the game falls through. However I think if you could still incorporate this if you simply had a friends feature and whatever name they decided to greet you with would appear in your friends list and contact this player. The same player could have 10 different names associated to him by 10 different players and they would all be able to message him/her still.
With regards to the bounty system this could also be worked out, in that if you place a bounty on somebodies head then presumably they would be flagged for their bounty as soon as a bounty hunter saw them. As such their name would appear as either ??? If the person who placed the bounty did not know their name or as the name the person who placed the bounty knew them by. Either way they would be automatically flagged to any hunters and the bounty amount would be shown as well.