is there anything out there that is like a cough button for guitar? A stomp box that only mutes when your foot is on the button so it's only momentary and not latching
I dont know of one commercially produced, but this would be very easy to DIY. A 1590A enclosure, a SPDT momentary switch and 2 1/4" female jacks, and you good to go. I suppose if you wanted to make it more complex you could wire in an LED with a battery that would come on when the mute action was performed.
if you bought a momentary switch, you could plug it into your midi board and then attach it to a volume block at the beginning of your grid