The following media keys are listed in the "Quick Input, Variable Keypress and Hotkey Key Indicators" section:
VOLUMEMUTE - media volume mute key
VOLUMEDOWN - media volume down key
VOLUMEUP - media volume up key
NEXTTRACK - media next track key
PREVTRACk - media previous track key
STOP - media stop key
PLAYPAUSE - media play/pause key
However, they are not listed in the "Key State Token Parameter Values" section, despite testing showing that all of them do in fact work with the "{STATE_KEYSTATE:}" token.
Also worth noting: "PREVTRACk" appears to be a typo, as it does not register keypresses while "PREVTRACK" does.
EDIT: As an unrelated (but log-related) side note:
The functionality of "{STATE_KEYSTATE:}" accepting/automatically stripping square brackets does not appear to be documented.
EDIT#2: Fixed with v1.8.5.6