A follow up to myself.
I tried to assign a USB device to my Xpenology vm using the following command.
qm set 100 -usb0 host=08bb:2704
After rebooting the vm and trying to play a song, I indeed got sound out. BUT...
The sound level was very low and very noisy. Worse, when Audio Station tried to play second song in queue, Diskstation webui and ssh got unresponsive. I tried to unplug the DAC and reboot the vm, but it made the host unresponsive too! I had to push and hold the power button of the machine.
So it works, but no.
What's wrong?
I wish I could at least test it on a bare metal setup to see if Xpenology could indeed use that DAC correctly.