This was an old trick for Drupal 7. By adding a settings.local.php file, we could set the 'file_private_path' and this would automatically override the config settings. I tried that tonight, but after importing the active site config, the private path is set to the value in the config files.
```$settings['file_private_path'] = 'private';```
Any way to override that for local?
As an addendum to this answer, part of my work tonight was writing a sync script using BRUSH. I have opened an issue there asking about enabling variable-set commands in brush, but that is probably also related to the question above.