I just worked this up as my PARTY page. with this the volume and lights controls on the right control ALL ROOMS

Each room shows active source name so that you can see if something is currently on in a room before you change it, the button 'glow' shows if the room is on/off and for the source just shows which source is selected.

The activity list on the left when PARTY is selected, only does a page flip to the source control page.

Individual volume control under each room name. This isn't huge job so I can fit it hear, otherwise I would have a separte popup with all the room volume controls simliar to what Sonos does. 

The bottom list of buttons will be party mode options. For example one will send APPL TV Video to ALL TV's plus a couple global lighting scenes like ALL lights 25%. etc. Basically just custom to what the client wants. Maybe a couple quick options to send a source to ALL rooms.