I think I’m not 100% clear on what you’re creating. Some kind of sound installation?
If you have a 7.1 audio device, the audio output per speaker is a discrete audio channel. Just because it’s a 7.1 WDM doesn’t mean you have to treat it like anything but 8 audio channels. They can go to whatever speaker you want outside your computer.
Submix Effects are pretty easy to create and it’d be simple to create submixes that mix their audio into a single channel. Knowing your output hardware configuration, you could anticipate 8 channels, etc. Literally giving you 8 submixes, one for each channel, all feeding into the Master Submix.
Then any sound you wanted to play out of a specific speaker, you could just route that sound to the corresponding Submix.