I just slapped this together...
Normal party mode view. Highlight your rooms, choose a source then set a preset volume that affects every selected room.
If you want to adjust a zone individually you press and hold the room button and a small volume box appears (in my example i have it appearing over the selected room. You could just do 1 volume box off to the side and use it for all the zones). Adjust volume and then press close. Or you could make it time out. I prefer a close button.
