These are the new Airplay device properties accessible via AppleScript:
active (boolean, r/o) : is the device currently being played to?
available (boolean, r/o) : is the device currently available?
kind (computer/AirPort Express/Apple TV/AirPlay device/unknown, r/o) : the kind of the device
network address (text, r/o) : the network (MAC) address of the device
protected (boolean, r/o) : is the device password- or passcode-protected?
selected (boolean) : is the device currently selected?
supports audio (boolean, r/o) : does the device support audio playback?
supports video (boolean, r/o) : does the device support video playback?
sound volume (integer) : the output volume for the device (0 = minimum, 100 = maximum)
Notice that all the properties are read-only (r/o) except for selected and sound volume, meaning only those two properties can be changed; the others can only be polled.