The tool is Master Muter.
In Renoise you can mute any of the tracks except the master track. If, for some reason, you want to mute the whole song, muting the master track would be ideal. If only.
To get around this, Master Muter adds a gainer device to the master track, sets it to total silence, and deactivates it.
The tool has its own OSC server. You can toggle that master gain device via custom OSC messages.
It’s a first release, so the usual caveats apply: May have bugs, may not work, might get a name change.