This is something a lot of us have wanted for a long time. Now I’m not sure if this was a change in Plex, or a feature developed from Amazon, but either way it works now. Albeit with limitations.
Now you can’t use the traditional syntax of “…play music on [speaker group]”.
With that said, under your device Groups, you have the option of setting a preferred speaker for your group. For your preferred speaker, you want to choose one of your multi-room speaker setups. So for example, in my Kitchen group, I have the preferred speaker set to play on my first floor speaker group which includes 3 other Echos on the first floor. So when I tell Plex to play music from my kitchen Echo, it plays on all 3 Echos on my first floor multi room music group.
It seems to work pretty well. Another limitation is that you can’t skip tracks from any other Echo than where you started the command, but that’s not a big deal for me.
Just thought people might find this useful.