Is it possible to bind an action to when the meta key (in my case the "Windows key") is pressed by itself in addition to combinations / sequences it's a part of?
Theoretically, yes; you would use modifier 0 and look up the XK_* value for the modifier key In practice, the X server might behave somewhat oddly because the passive grab on the modifier key might interfere with the passive grabs on the modifier+key combinations.