I’m not exactly sure why you want to do this. If acf-json is a symlink in the template directory and you add get_template_directory().'/acf-json' as a path for ACF, writing a file to the symlink will write the file to the path in the symlink. This should be handled by your server’s file system, or PHP. The only thing I can think of that would prevent this is if PHP does not have permission to write to the path that is symlinked.
