imt, I may not be understanding your question. Using a DMS-1200 as a mutli-channel amp is using it the way it was designed. It IS a multichannel amp. To me, its pretty much the crown jewel of a TC system. Pretty flawless and easy to work with. (aside from not being able to group zones via a macro, yet!).

If your goal is to power speakers, have sonos as an input and use the sonos app, I'm not seeing where an issue exist? Am I'm missing something? Maybe you are wanting sonos amps and use grouping in the sonos app?