[popcornmix]

Edit: Looking at the text of the notification - "New device configured: CEC Adapter" - I wonder if it's xbmc that is kicking out the notification,rather the CEC peripheral/driver itself.

I think that is the key point. The notification shows a CEC adaptor is detected, not a CEC device.
Initially that would have been added to support the Pulse-Eight USB CEC dongle, and the notification shows that the dongle is detected okay.

For a USB dongle, that makes sense. You want to know it is detected okay. If you wanted it disabled, you'd presumably unplug it.

But raspberry pi you can't unplug it. It is always there.
I can check if returning fail from vc_cec_set_passive stops the notification, and then make hdmi_ignore_cec cause that to happen.