For future reference: if you have an Onkyo AV receiver, these are picky about the CEC logical address of the player. If you have a default installation of Kodi, the default type is "recording device", and the logical address is 14 -- this is rejected.

If you change the CEC device configuration, which can be found in .kodi/userdata/peripheral_data/ to use a device_type of 4 (which maps to a logical address of 4), then the receiver will start sending key events to Kodi.

You may also need to set your remote control mode to 32910, by pressing the Display button in combination with the input source selector your Pi is connected to for three seconds, and then entering the code.

This setting is not available from the GUI, so you really have to change the XML file.

Next step: finding out why Kodi ignores these key presses. :(