Thanks for the response, hcs. So stereo ogg vorbis is encoded sort of like the joint-stereo mode of MP3.
However, for audio with more than two channels, I found this quote from the vorbis mailing list: "<i>Currently, the Vorbis encoder use a mix of lossless stereo and point stereo for 2 channel encoding. If there are more channels, each channel is coded independently, hence they are uncoupled.</i>"
So, could this mean that multichannel ogg vorbis files can be losslessly decoupled to multiple independent vorbis streams?
Again it is not trivial, but it sounds like it would be possible in that case. The Vorbis spec has all the details needed but it is hard to wrap one's head around. I suggest searching for "audio_channels", this necessarily shows up in most places where you're doing something for each channel. With no coupling you get to disregard the mapping coupling (2.c.ii.A) and section 4.3.5.