I figured it out. I copied /mnt/mmc/MUOS/info/name/content.json over to /mnt/sdcard/MUOS/info/name/content.json (it didn’t exist on the SD card on my device) then added the entries there. It didn’t work to add them to any of the pre-existing json files, nor to add them directly in /mnt/mmc/MUOS/info/name/content.json.
I’m sure this is quite an edge case, but if the existing .json files in /mnt/sdcard/MUOS/info/name aren’t used, it might help avoid confusion like mine if they were removed or marked as inactive somehow.
Thanks for setting me on the right track!