Command-L does not work for me to maximise the window from the dock. I am not in the Finder and I have tried both with and without full keyboard access.
What I have noticed is that Command-L will maximise the window, but only if that shortcut has a function in the app itself. For example, in Safari, Command-L opens a location. In System Preferences, Command-L will show all preferences. In both of these apps, the window will maximise, but in others it won't.