Shoutbox

A way is with a moded client that both users would have to use...the alternate typing name could be sent over to the other client via a direct connection...Who's going to make another client just to do this, I doubt many...
Another way is somehow hooking into Messenger and changing it at the last minute from info that was sent somehow earlier...probably a text message that the other computer recognises...
I doubt this will be made as there isn't really a demand for a feature that really has no use, or purpose...