Update on this project.
Basically, cancelled because I found an easier/better way.
Bought a Sonos Play speaker (actually got 2 - the price is falling nicely). Connected it up to Stringify using SmartThings. Placed it right next to a Google Mini.
Bingo, I can send any message I like to the speaker. Such as "ok Google, do this and then do that". It works brilliantly!
Now I can effectively have Google do my bidding from Stringify - even run routines, broadcast messages around the house automatically when I leave work, start/stop Spotify, or suchlike - eg. things you cannot do through an API.
Nb. I got the Play rather than the One because the latter has voice assistant built in (Alexa, which I don't use, and Google Assistant to come this year, maybe). But I don't know how they will implement it. I didn't want to find that the speaker would start to talk to itself or refuse to talk at all. With the Play, I just put a Google mini right next to the Sonos and it works beautifully.
And here a very simple flow to turn Sonos music on using Google Assistant (even tho Google Home doesn't support Sonos directly right now) ...