This will be very difficult to implement because detecting the Command key on Mac OS using JavaScript is done very differentlydetecting the Command key on Mac OS using JavaScript is done very differently. I don't think it's worth it.
Also, it's likely that the OS will capture Command-key equivalents before the application does. It's very rare to be able to use ⌘-Any key combination on websites. I don't think most users will expect it.
Besides, should you just be writing markdown? :-)
EDIT: I can confirm this does work in Chrome for Mac... Meaning I change my position. This should be fixed to be made uniform. Either support it uniformly, or don't.