After a week of long research and explorations into ways that "should work" (according to the technical specs) I have found a way to play greater than 8 discrete audio channels from a computer to a Dolby Atmos Receiver over HDMI (without a $1500 encoder app).
Windows 10 "Creators" and above can encode Dolby Atmos on the fly to HDMI sound devices. It's only exposed to applications via library calls, however, not a simple sound device that existing sound apps / players can use. It does cost $11 via the Dolby Access app in the windows store.
So, it will take some work to code the example into something useful for us, but this is really good news as far as sharing and producing for yourself upmixed music into the newer immersive speaker layouts.
Windows 10 plus Atmos supports speaker layouts up to 8.1.4.4, meaning up to 17 speakers, 8 around you, one lfe, 4 above you and 4 below you. There is also support for sound "objects" that can move or be "rendered" from anywhere in the sound field. Not sure the object stuff has any application for upmixing (except maybe to define multiple listening positions within your room) but it's there.
Also holding back on doing anything the with the "below" speakers, as I don't think anybody has a receiver that supports such a setup. But, if teach Spec (or a new name?) to work in 8.1.4, Dolby Atmos Receivers should automatically map the output into whatever layout you have. 5.1, 7.1, 5.1.4, 7.1.2, 7.1.4, 8.1.4, etc.
Some of the learned things along the way (that turned out not to be helpful):
There are commercial cloud based encoding services (pay as you go vs. buying an encoder), and reasonably priced for audio only, but none offering Dolby Atmos or DTS X (limited to 7.1).
Dolby Atmos is actually delivered over Dolby Digital Plus, which is an open spec, known as e-ac-3, or ec-3 (extended ac3), which does support up to 15 channels, but no commercial or free encoders go beyond 7.1 (and it's not clear that any free ones actually go beyond 5.1).
Published in Annex E of ATSC A/52:2012,[1] as well as Annex E of ETSI TS 102 366 V1.2.1 (2008–08), published by the Advanced Television Systems Committee.
ffmpeg being an example of a free encoder that can do dolby digtital plus, but there are multi year open but reports on lack of support for 7.1 (let alone anything more). Aften is another, but again 5.1 only and not active development.
Cheers,
Z