Your suggestion is something that will be implemented thoughout the entire of Messenger Plus! once it is possible. Now Windows Live Messenger uses custom names, and can organise them better (although Plus! made them more customisble) there is no need to have this feature in Plus! anymore.
However you could argue this feature was half-coded, as no API functions were added. Unfortunately custom names are stored on the server side, so there is nothing Plus! can do until the Messenger team add the function to the API
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