I created passthrough branch at
https://github.com/jernejsk/OpenELEC-OPi2/tree/passthrough
There are two things to consider:
1. kernel drivers treats samples as 24 bit but kodi as 16 bit. Probably 24 bit hack should be removed in kernel.
2. kernel patch linux-93-hdmi-fix-raw-output.patch might be needed, just remove ".disabled" from file extension to test
Please build, test and report