Shoutbox

It seams simple to look at, i mean its only a load of text, but in practise it would be very difficalt. Because you would have to alter what the person at the other end see's. And to my knowlage isnt the information sent from the server to the client. Therefore you would have to intercept this information and alter it, which would involve packet sniffing, which is illigal on networks you dont own i think.
So the only way to do it is if the contact had a plugin that requests the custom name from you. And that would mean that all you contacts would have to install this plugin. And you cant garatie on everyone haveing it, Like plus or Filetransfer plugin.