In a similar vein, I was delighted to discover that my rather elderly Audiolab M-DAC (used as a pre-amp) exposes its master volume control over USB digital input when plugged into a RPi.
Although I don't use USB for audio (rather buggy) the control interface works perfectly.
I duly created a websockets API that allows me to remotely control the volume over wifi via a phsyical rotary controller. Allows me to conceal all the hifi equipment in a cupboard upstairs but remotely control it from a knob downstairs in the kitchen.
Like the OP I also implemented volume limits to prevent accidental damage to the speakers (primarily from twiddles by little fingers)....works a treat!