I'm looking for a way to put the menu bar on the same line of the title bar if the latter fits in the former.
Is there anything like that out there?
No, I don't think there is any window manager out there that does this. This is because the menu is part of the application and the window border is usually handled by the window manager.
There is only an idea on ubuntu brainstorm: http://brainstorm.ubuntu.com/idea/9417/ You can promote this solution.
That would mean that someone has to integrate the menu bar into the window manager. There is a project out there called gnome global menu. They took the menu bar and turned it into a gnome panel applet. So it should not be too hard to do this. As for now I don't know of any window manager that has done this.