I think I've just realised something; you are creating a web page, and you don't want the 'click and hold' menu to appear. It's not that you are a mac user who wants to turn it off, but a web author who doesn't want others to use it.
Assuming I'm right, then the answer is much simpler. The 'click and hold' menu is exactly the same as the right click menu in Windows and Linux (and OS X if you have a 2 button mouse). It's proper name is the 'context menu', which is why returning false to context menu requests causes it to be disabled.
There is no 'Mac Javascript', just Javascript - it's the same on all browsers and all platforms (with the notable exception of IE, although it's getting a little more standards compliant). Disabling the context menu on the mac will disable it for everyone else too.