Peter,
Ilike to integrate it in rooDial ( rooDial a Wireless Volume Knob for Roon with Microsoft Surface Dial - Audio Products - Roon Labs Community)
At the moment I can set a Roon enabled device to standby. My idea was to offer an option to turn on- and off the USB power of the PI. So you can connect a cable to the USB 5V power and trigger the amp with it. Most amps are fine with 5v instead of 12V.
I already proposed it here rooDial a Wireless Volume Knob for Roon with Microsoft Surface Dial - Audio Products - Roon Labs Community but there was not so much feedback on my idea so I did not implement it yet.