Seeing as no-one answers, I will try. Assume well meant attempt to help, not expert knowledge.
If you connect the IdPi to a USB DAC that supports multichannel I believe that will work, theoreticaly at least. The IdPi onboard DAC (Soundcard) only has Stereo options visible, so I would assume multichannel out of HDMI or optical digital port would not work, only stereo. The other question would be: if you do connect to an external multichannel capable DAC would the IdPi have enough numbercrunching capability to push multichannel out to it without faltering (stuttering)? I am not too sure about that. The Intel NUC Id manages Multichannel very well. Might well be a bit limited on the IdPi.... which is a great piece of kit, but it is not a monster cpu. Hope I havent confuised the issue.