Well, stone the crows.
I have found a solution to the use case, using an entirely unexpected approach (indeed, one I was not even aware of - but then I am very new to Wix), when I was not even looking for a solution. Anyway, I started to play with Dynamic pages for our journal (having initially set up what was effectively a dynamic page manually), when I had a thought. So I created a dynamic page for the authors. And on that page, I put a repeater to display journal information. And I wired it up so that the author was used in a filter for the journal, and, to my amazement, it worked. Now, you have to select the author in that widget on the top right of the screen, not a control on the page, which people may be confused by, but it works.
I am not sure what the users will make of it, and I still have to investigate how to get to the page in a manner that users could understand. I don’t think it is as good as having a drop down on the page, because of navigation issues, but for the time being, I am going to declare victory, and wander off with a smug look on my face.
Edit. Not looking quite so smug, now. In edit mode, when testing the site, there is this handy little widget top right to navigate around the chosen authors. On the live site, there isn’t. Am now spending even more time trying to sort that out.
Sigh.