I am boxing up an arcade machine and i would like to implement volume control via keycombinations (we are using Retrogame script currently)

Soft shutdown is to stop SD corruption
and
System volume allows me to skip all the bizzarre rom-by-rom volume controls and not have to work out a complicated amplifier mount allowing potentiometer access

I imagine something along the lines of "hold a+b+c and Joystick+ or - to adjust sound" which is going bumping amixer up or down.

Has anyone already done this?
Do tell!