Perhaps this Plus! registry key will do the purpose:
(
http://www.msgplus.net/help_registry.php )
Key: HKCU\Software\Patchou\MsgPlus2\your@email.com\Preferences -> NameOnSignin (String)
Description:
If not empty, Messenger Plus! will use this value to change your name to a default one every time you sign-in on a specific computer.
You just insert this key with proper value to every computer you use to sign in .Net Messenger service with