Matt,
I need to get my head around the multiroom thing for this project:
I need to go back through the command sets (HTTPAPI, TCP/IP, UART, DLNA/UPNP) and it seems BLE is a new one too 
and with the help of Arylic figure out all the Master <> Slave Stuff. It’s definitely on other side (The MCU with a TCP/IP Service) but how it is presented to the various command sets needs some work to figure out. Also worth checking out Linkplay https://www.linkplay.com/modules-wi-fi-2. The A31 is the board used for a lot of the Arylic Products.
The Linkplay detailed documentation is poor (it’s more of an OEM thing IMHO). Arylic are very open, progressive & customer orientated. However there are limitations of what they can do with the A31 module - some functionality and peculiarities will always be dictated to some extent by Linkplay.
So I think the multiroom Master <> Slave Stuff was/is/could be a Linkplay thing. Just wanted to warn you if you weren’t aware.
Enjoy playing, the devices sound great for the money and offer plenty of functionality to boot 
Keep going with the observations, questions etc. etc. 
Kind Regards, Kevin