I forgot about that check, that is also by design 
But one of the advantages of doing scripting is that you can manually bypass the check with a quick modification.
To do that, close Messenger, head over to \Program Files (x86)\Messenger Plus! Live\Scripts\WLM Taskbar Features, right-click main.js and choose Edit.
You should now have Notepad open, scroll down to the
function OnEvent_Initialize(MessengerStart) function, and change the line
if (IsOnWin7OrAbove())
to if (!IsOnWin7OrAbove())
This reverses the check.
Save and exit, and then start Messenger.