This adds two custom actions. The first being "fade master by 1dB then wait 0.1 sec", the second being "Fade master to minus infinity" which calls the previously described action 150 times. To use it, assign a keystroke to the "Fade master to minus infinity" action. Probably worth pointing out that - at least on my system, Reaper is unresponsive during the fade, but playback doesn't seem to get interrupted.