Objective-c – Add Subview to NSWindow Titlebar

I want to create a custom Window Toolbar but without using an actual NSToolbar (because it doesn't behaves right in some cases for my needs and I can't style it the way I want), therefore I need to add a subview to the Window titlebar.
It should look like the following:

A posible way would be:

[[[_window contentView] superview] addSubview:_titlebarView];

but this has the problem, that it causes a warning on Yosemite (NSWindow warning: adding an unknown subview: <NSView: 0x608000122b20>) so this doesn't seem to be the right way to do it.

Currently I am adding the toolbar view to the titlebar view:

[[[_window standardWindowButton:NSWindowCloseButton]
    superview] addSubview:_titlebarView];

but this feels wrong too.

Since my App only targets OS X 10.10 (Yosemite) it would be fine for me to use Yosemite-only API's, so I had a look to NSTitlebarAccessoryViewController and I managed to successfully subclass it and using it with NSWindow's addTitlebarAccessoryViewController: method. With this approach there remains one problem, the possible values for layoutAttribute are:

• NSLayoutAttributeBottom
  Adds the view to the bottom of the window's titlebar, while preserving the view's height.
• NSLayoutAttributeRight
  Adds the view to the right of the window's titlebar, clipping the view to the height of
  the titlebar.

The NSLayoutAttributeBottom is not suitable for me, since I need to position things right
at the top of the window titlebar (see screenshot above) and center the traffic lights.
NSLayoutAttributeRight would be exactly what I need, but I can't find a way to increase the titlebar height, so the view is clipped to the titlebar height, making it useless for me. (Note: Using a textured Window and changing the content border doesn't work, since it will not change the actual titlebar height)

What is the recommended way to do this? I would prefer to go with the official API's and use the AccessoryViewController approach, if it would be possible to have a view larger than the default titlebar.