As per the above, you need to think about how HDMI works.
Set the Oppo to output multichannel LPCM over its HDMI output and disable secondary audio. Set the HD621 to 6 channel mode. It will then tell the Oppo to send just this over HDMI.
The HD621 will then strip out the audio from the HDMI stream and transmit it to the G68 as 3 x SPDIF into an MD input.
I have this exact setup and it works perfectly. The only "trick" was to understand which outputs on the HD621 correspond to the MD channels on the G68 as they are marked in different ways.