Ah. Using the source, I see an option was added (the value of the git commit log!) to support page-layout being associated with a specific editionID. This solves the problem in my context.
But I do note that by trial and error I found the editionID name has to be in a list, which I thought a little strange for something that will only have one item.
\setOption page-layout.conditional-breaks.score-id #'(sq)
Andrew