Thanks, this is definitely the way to go. One problem may be that on later MacBooks, there is no option in Security and Privacy for anything other than 'App store' or 'App store and identified developers'. I used a 2013 MacBook Pro in the end, running an older version of OSX that offered an option for allowing all apps. Even checking that didn't make the driver show up in the Naim app.
So I went to https://www.silabs.com/product...t-bridge-vcp-drivers and downloaded the driver from there, making sure I used the legacy version. With the 'Allow apps' setting at allow everything, the serial driver did finally show up and I could do the update.
Since both MacBook Pros had a Silicon Labs USB to UART driver loaded before I did any of this, it raises the question about why it all didn't work out of the box. Is is that the Naim app is looking for the wrong driver (the manual cites the CP2101, while both Macs had the CP2102)? Or is it that Silicon Labs don't sign their driver (which seems less likely, because with all security turned off, you would expect the laptop to see it)? I can understand that on the 2018 MacBook Pro, the lack of an option to turn off security completely might be an issue, but I can't really believe that Silicon Labs don't sign their drivers.
I have reported the issue to Naim, but we know how long they take to service user requests, so I am not holding my breath. But with something as predictable as a MacBook Pro, with a restricted range of hardware, software, and operating system versions, this doesn't exactly fill me with confidence in Naim. Thanks everyone for the support, without you, where would we all be?