MySQLDumper pre-selects the detected standard encoding of the MySQL server.
Nevertheless you could overwrite this to fit your special needs.
Open the file filemanagement.php and change line 23 from
$dump['sel_dump_encoding']=(isset($_POST['sel_dump_encoding'])) ? $_POST['sel_dump_encoding'] : get_index($config['mysql_possible_character_sets'],$config['mysql_standard_character_set']);
to
$dump['sel_dump_encoding']=(isset($_POST['sel_dump_encoding'])) ? $_POST['sel_dump_encoding'] : get_index($config['mysql_possible_character_sets'], 'tis620');
This will make the character set tis620 pre-selected and your customer can now "forget" to change the encoding.