When a Raspberry Pi is connected to a CEC bus with HDMI it sends an active source cec message on boot. This can be modified in
/boot/config.txt with setting
hdmi_ignore_cec_init either 1 or 0 as described in the RPI docs.
Now what is the byte content of the message send by the RPI, so one could replicate it with tools such as
echo "tx <rpi-cec-message-sent-on-boot>" | cec-client -s -d 1 ?
The easiest way to find this out would probably to have a second Raspberry Pi monitoring the bus while the other one boots, but I only have 1 RPI, so I can not test it myself.
This is a summary of my full problem, I already posted here.