There are quite a few different pico remote that lutron makes, my understanding from the Hubitat documentation is that all picos are roughly the same, and it just have different numbers of buttons, that you can map arbitrarily.
A. is the above accurate?
B. Is there a setup that allows you to map a pico, lets say the Pico Audio to a specific Cast Speaker group? with "middle/favorite" associated with a specific playlist (e.g. I'm feeling lucky playlist from Google Play Music / Youtube music)
I'm afraid not, but I'm commenting as much to see responses as to answer your question re: cast options.
You're correct about the picos, though. They're pretty much all the same. Some are just more expensive because of supply and demand.
The problem with cast groups and the way you want to use them, is that you can't send a command in any way to control the apps to initiate the playback. It's really the music apps themselves, like Spotify, who initiate the playback, and then cast to the groups. You could create an action that speaks a command, "hey Google, play my favorites on Spotify on master suite speakers," which I've tried and works well enough, if you have a speaker close enough to another speaker to hear it. But it's kludgy.
Like I said, though: I'm hoping things have progressed since last I was trying to do similar things, so I'm anxious for some response to show how out of date my answer is. Good luck!
And by the way, with cast-web, (which, if you're not already using it, I would highly recommend), you COULD do this with a lovely locally stored music library or playlists or some web radio. But none of the "services" like Spotify, Pandora, Google play music, etc.
HA! so basically I need a little soundproof box in the garage with two google home mini's to talk to each other to accomplish this?
unfortunately even that might not work since we have multiple users with speaker ID in the house "my" I'm feeling luckily playlist would only work if I were the one that spoke the command...
Let's say I worked at Google, what would need to change to make this possible? to be service agnostic, you'd just need to pass a commands to the Assistant, via...what? I don't know how habitat is currently integrated with the assistant. happy to log a bug/feature request if someone can describe what is needed. I'm imagine its like typing to The Assistant (via Assistant app on phones) but programatically.
Lol. Not the first to think about the sound proof box concept. 
I think the issue is, Spotify and Pandora and such have APIs that they open up to the big boys, and individual users or smaller developers can't get that access thus far.
Think of a Google home hub (or Nest Hub as it's now called): you make a voice request, and it calls on Spotify APIs to connect via the credentials you've stored in Google Home, start a playlist, and it brings all the information of that track to your screen. That's not part of the Google cast capability--it's actually Google Assistant don't the work there. If you want to skip, you hit next track on your Google home hub, at which point Google is actually asking SPOTIFY (again, through their APIs) to skip the track. A "next track" command through the Google cast APIs is not tied to any function in particular, and doesn't pass through to Spotify. Again, "hey Google, skip track" is asking the Google Assistant to use its feature set to access Spotify APIs to make that happen. Google has their own interface that accesses Spotify for this. Google cast has no such connection. I hope I'm making sense.
So really, what you'd want/need for this to happen is for Spotify--not Google--to open up API access for you. And man, do I wish they would.
The good news for Sonos users is that they have a little more of this functionality passed through their devices. So a request from you to Sonos is passed along as the same request to the service. Sonos acts more like a controller/hub for your music services, whereas Google cast acts more like a speaker that will play them if you push it to them from outside.
This is a great explanation.
Download the Hubitat app
