Neil, I think that should work on any Mac. It's basically all that AppleScript did back in OS 9. But back then we had
both Startup
and Shutdown items. OS X only provides Startup items. The advantage of the script is that one doesn't need to remember to use the Mute key. One actually need not even be present for the mechanism to work.
The other part of the 'trick' is an AppleScript that returns the sound volume to whatever is 'normal.' That part is still available in X, of course.
I suspect that someone must have written a AS or even an Automator that will run at a particular time and shut down the Mac. Before doing that, of course, it could easily turn the sound OFF. Someone needs to search for something like that. There are many who have a Mac in the same room that they sleep. Keeping the 'noise' level to a minimum should appeal to them!
Another for Psst!