Does any HDMI device send commands on CEC pin
You'd need a detailed manual for the set-top box (or better still, call tech support) help to figure out is CEC control is an option that needs to be enabled in the IPTV box.

CEC is a 1-wire interface and this Arduino thread might help you: http://forum.arduino.cc/index.php?topic=22327.0

If it turns out that you need to build your own external functionality to implement CEC control perhaps you could build an IR RX to use the commands from your IPTV remote. Or you might even be able to implement the TV CEC commands based on the IPTV box going from active-to-standby based on mains current detection.