The problem is that is something global (Inbox is also maximized). So I think can be put both things (the global menu item and the content maxmin button).
Classes you have to look into:
MaxMinP
and:
*ClientActions
The "Maximize the workspace" is very useful (especially in the group space) and it'd be convenient to encourage its use. It's difficult that people will see it in the "More" menu, or even relate it to the document they are editing. It'd be better to have a "Maximize" button typical from any window in any operating system. Maybe right now it can be next to the (i) (information button) that we see when opening a document.