Control one Meridian endpoint using another

Hi there, I wonder if this is possible somehow…

I have a Meridian 818v3 and have it configured on my ROCK-based server by both USB and ethernet. I prefer listening via USB so I can play DSD using DoP, but in this configuration I get no comms control of the 818 for volume, and find myself switching back to the ethernet based endpoint in the roon remote for volume control, then back to USB for music selection.

I see in the device setup that you can select a different method/device for volume control (and see lots of Home Assistant entries given that integration is present), but I cannot seem to get this to work or find much in the documentation as to how to use this setting.

It would be great if I could make this a little more seamless - any thoughts greatly appreciated.