ok, I was bored so I made the plugin for you:
commands are:
/xnetsend computer message
/xnetsendgui
Included a little gui with a flood option.
Also it had 2 methods of sending the message the first option, wich is default will use the standard net send command, the second method uses netapi32.dll and returns error messages, this method will send faster but when a computer isnt found msn wil freeze for a couple of seconds. Thats why I included the standard "Command line" method.
The method you choose will be saved in register so next time you start msn it will use the previous method you used.
I hope this works for u, if you have questions / errors / suggestions let me know
have fun with it