Shoutbox

messengerdiscovery http://www.messengerdiscovery.com/
does that with msn 6.2

it would be a cool plugin if one can be made in plus! though..

Edit:
for now with msn 7 beta.. you can use stuffplug:
http://www.stuffplug.com/
in it there is a function to use a "chat only" name you could put in the user's name and say something "pretending to be" them. but it would be pretty hard/annoying switching back and forth.