A soft-mute (a python script that calls alsamixer) is probably the easiest option. Find any free GPIO pin on your Pi, and a Ground pin, then connect an SPDT switch across them so that in one position it shorts that GPIO pin to ground.
Set up the GPIO pin as an input/pullup and it will read 1/HIGH until the switch is closed. You can then use an event detect in RPi.GPIO to wait for it to go 0/LOW and trigger the mute, then wait for 1/HIGH to umute.
We've got a product in the works with push-buttons that might be just the ticket. You've actually given me a great idea for a useful example to ship with it :D