It's not true that you are limited to 7.1 on Windows. The windows audio stack is limited to 7.1, but there is nothing preventing you from using ASIO to output to a DAC with more then 8 outputs. I do this very thing right now to a Focusrite 18i20. I output 10 channels (7.3).
The only problem is that the decoding software must be ASIO capable, and smart enough to decode the atmos bitstream. I suspect JRiver MC is the closest at the moment. They don't have atmos decoding, but they are certainly capable of outputing to ASIO DAC.