After a bit of messing around I have managed to get my Apple TV to control a TV that supports CEC. This is via an HDMI splitter that also sends output to a (non-CEC) projector.
So when I wake the Apple TV the TV automatically wakes and displays the ATV media - great. If I power up the projector it also displays the media - so I have mirrored displays. Note that if I only want to use the projector with the ATV the TV wakes as soon as I wake up the ATV.
My problem is that, due to the bi-directional way in which CEC works, if I turn off the TV the ATV also turns off, as does the signal to the projector. Therefore I can only watch the projector if the TV remains on. There doesn’t seem to be a way to prevent the TV turning the Apple TV off in these circumstances.
Maybe my only (longshot) option is to provide feedback to Apple requesting an extra setting to prevent the ATV being controlled by the TV?
Or is there a pin in the HDMI cable that is only for the TV sending a control signal to the ATV and so disabling it would fix my problem? Update: apparently not:
“It is a single-wire bidirectional serial bus…”