Ah... that could be done (the DLL itself doesn't need to be changed though; only the script itself. Because you could use the same parameters as the SendTo script needs; the only thing to change is how you would handle those parameters in the script itself)....
However. All that isn't realy needed.
If you have polygamy disabled, you don't need any script at all for this; Messenger already supports it
. Simply open Messenger with the following command:
"C:\Program Files\Windows Live\Messenger\msnmsgr.exe" /sendim
and the 'Send instant message' window will pop up (provided Messenger was already running and you were signed in of course).
Aka, create a shortcut with the above command line and parameter and place that shortcut in your Start menu.
(though, of course make sure the path is correct)
Other similar command line parameters you can use:
/available Sets your status to online
/busy Sets your status to busy
/away Sets your status to away
/invisible Sets your status as appear offline
/signout Sign out
/signoutal Sign out at all logged in computers
/sendim Opens the 'Send instant message' window
/msn Opens the MSN home page
/email Opens your Hotmail inbox
/profile Opens your Windows Live profile