You make this into a script that you can call with a single command or even just a shortcut on your desktop.
Get a list of all Zones on your JRiver machine.
http://<server IP>:52199/MCWS/v1/Playback/Zones
Now you have 2 options, web requests or MCC commands.
For MCC
https://wiki.jriver.com/index.php/Media_Center_Core_Commands#Specifying_ZonesWill help you figure out how to 'target' a zone with a particular command.
We'll make a .bat script that has x number of "play/pause" MCC commands that run right after another, one for each zone.
So now when you want to Play or Pause all zones? You're just going to run this .bat script.
Similarly for MCWS, we'll use the play/pause MCWS command, just as many as you have zones, specifying the zone in each command of course. With something like Curl (included in windows now IIRC) we can use a .bat script that has all these commands in it, to make the MCWS web-requests one after another in a row.