Shoutbox

It does permit the action. Follow these steps:

1. Launch an instance of Windows Explorer, UAC-elevated.
   
2. Exit any running instances of Messenger.
   
3. In that Explorer, browse to %ProgramFiles%\Windows Live\Messenger.
   
4. Create a backup of the original msgsres.dll.
   
5. Open the DLL in Resource Hacker.
   
6. Expand the 4010 section.
   
7. Click the "947" resource, and the language id resembling to your Messenger language (e.g. "1033" for English).
   
8. Click "Action", "Replace other resource...".
   
9. Press the button "Open File with New Resource...".
   
10. Search the binary UIB file for 947 on your HDD, and open it.
    
11. At "Resource Type", enter "4010".
    
12. At "Resource Name", enter "947".
    
13. At "Resource Language", enter the number you clicked in step 7.
    
14. Press "Replace".
    
15. Now save the DLL file with "File", "Save".
    
16. Start Messenger to see if the replacement succeeded.

If there are any problems, reply with the error that occurred.