Update:

Managed to get the updater working in FreeNAS using the following command:

su -m www -c '/usr/pbi/owncloud-amd64/bin/php /usr/pbi/owncloud-amd64/www/owncloud/occ upgrade'

Unfortunately, however, the update filed. Here's the last bit of the update process listed...

Updating <dav> ...
Fix classification for calendar objects
 Done
    2/2 [============================] 100%
Updated <dav> to 1.0.1
Updating <files_external> ...
Updated <files_external> to 1.0.2
Updating <files_sharing> ...
Updated <files_sharing> to 1.0.0
Updating <files_trashbin> ...
Updated <files_trashbin> to 1.0.0
Updating <comments> ...
Updated <comments> to 1.0.0
Updating <systemtags> ...
Updated <systemtags> to 1.0.2
Update 3rd-party app: files_antivirus
Doctrine\DBAL\DBALException: Failed to connect to the database: An exception occurred while executing 'PRAGMA journal_mode = WAL':

SQLSTATE[HY000]: General error: 10 disk I/O error
Update failed
Maintenance mode is kept active
Reset log level