Unfortunately Apple has access to APIs the rest of us don’t. Messing with the title bar is really hard.
The best I can suggest is making your window NOT have a standard title bar, and then placing the buttons yourself by calling [self standardWindowButton:X] for each of the close, resize, and miniaturize buttons you want. Then place your own document icon and title textField.
You’ll likely have to track when the window loses or gains key or main status and modify the buttons accordingly (Cocoa fetches new buttons each time this happens, not sure why). Whee! Good luck!