in custom_fields.php, on line 222 I've added:
<option value="text">Text</option>
In asp.shortcode.custom_fields.php I've added in line 60:
<?php case "text": ?>
        <input type="hidden" value="<?php echo $item->asp_f_hidden_value; ?>" id="aspf<?php echo $_in; ?>[<?php echo $item->asp_f_field; ?>]" name="aspf[<?php echo $item->asp_f_field; ?>]">
In search_indextable.class.php I've changed the line 355:
} else if ( ($data->asp_f_type == 'radio' || $data->asp_f_type == 'hidden' || $data->asp_f_type == 'text') && isset($posted)) {
Also in search_content.class.php on line 423:
} else if ( ( $data->asp_f_type == 'radio' || $data->asp_f_type == 'hidden' || $data->asp_f_type == 'text' ) && isset( $posted ) ) {