We can add Charms and Custom Buttons for pretty much everything in the file menu, but we still don't have one for "Close" nor is there any way to customize them as Dan mentioned. Should we just assume we won't be getting a Close button?
Charms require graphics, so customization of charms will always be limited. Buttons are just text, so they are easier to expand. v6 will have a lot more options for buttons. In fact pretty much every menu option will be capable of being a button. We've also added customizable hot keys, so you can make any keyboard combo trigger a menu item as well.