Congrats on a great app.
I've seen it written on the forum that multi-room, with control via Hi-Fi Cast, is only possible with Chromecast.
FYI and FWIW - It works on my Panasonic ALL devices (UPnP), and therefore will likely work on other Qualcomm AllPlay devices (speakers/connects) too.
Obviously it's using the featureset of AllPlay to do this - I've heard it referred to as re-streaming (from the Primary Renderer).
Just select the Primary device as Renderer in HFC
Grouping (is) and volume tweaks (are/can be) handled from the Panasonic Music Stream App.
... but queuing and transport control can be done from HFC.
I don't know if this works on other eco-systems e.g, Sonos, HEOS, MusiCast ???
Have Fun,
Shell
PS - Thanks for the 'findhdmusic' site too.