There's one more commercially available DAC board that is based on the same DAC chipset and also has no volume control: the Pimoroni pHAT DAC. There is a Volumio configuration file available for it and it seems like it's been tested already....
Based on a quick look at the Volumio online documentation, you will likely need to go into Volumio's "Playback Options - Volume Settings" and choose "Mixer Type = Software".
Thanks Mike! This was the trick. Selecting the pHAT DAC in the startup wizard of a fresh install of Volumio gave me the ability to control system volume. This will work for my purposes for now, but I'd still like to see Axiom add eventual support for Spotify Connect.
