How to fix RaspberryPi HDMI-DDC/CEC issue while keeping ESD protection?
You only need to cut/remove the diode that connects the CEC pin to 5V as it is the one that loads it down without power supply. That diode is dubious anyway, as during normal operations it would clamp at above 5V which is already too much for the 3.3V CEC signal so. But this may still not work as there are no mention about the Broadcom IO pin structure for the CEC pin. So if there are internal protection diodes or unswitched CEC pull up, it would still load down the CEC pin in a non-compliant way. Best circuit would be to just put a FET there as a pass gate that is off when no power is applied. You would not even have to remove the diode then.