I'm guessing that the problem could be due to my authPDO configuration not containing a particular SQL string that would be necessary to add new users
Correct ! Register action does not work unless the underlying auth plugin can add users. There is an explicit test in DokuWiki code.
, but I'm not sure what that might be.
Probably all the SQL statements mentionned in plugin:authpdo#additional_sql_setup
. There is an example for each of them. That should help you to write the statements for your database schema.
$conf['plugin']['authpdo']['select-groups'] = 'SELECT ... ';
$conf['plugin']['authpdo']['insert-user'] = 'INSERT INTO .... VALUES( :user, :hash, :name, :mail)';
// and so on