I have had a play and managed to get error messages working ...
Here is how I did it ....

Please note ... rather than 'taxid' I am using 'studentname' as our cart if for a school..
1. /includes/languages/YOURTEMPLATE/english.php
a. Find ' define('ENTRY_TELEPHONE_NUMBER_TEXT', '*');'
b. Under this entry insert
//
define('ENTRY_STUDENTNAME', 'Students Name:');
define('ENTRY_STUDENTNAME_ERROR', 'Your childs name must contain a minimum of ' . ENTRY_FIRST_NAME_MIN_LENGTH . ' characters.');
define('ENTRY_STUDENTNAME_TEXT', '*');
//

2. /includes/modules/pages/create_account/jscript_form_check.php
a. Find ' check_input("telephone", <?php echo ENTRY_TELEPHONE_MIN_LENGTH; ?>, "<?php echo ENTRY_TELEPHONE_NUMBER_ERROR; ?>");'
b. Under this entry insert
<!-- STUDENT INFO HERE -->
check_input("studentname", <?php echo ENTRY_FIRST_NAME_MIN_LENGTH; ?>, "<?php echo ENTRY_STUDENTNAME_ERROR; ?>");
<!-- END -->

3. /includes/modules/pages/account_edit/jscript_form_check.php
a. Find ' check_input("telephone", <?php echo ENTRY_TELEPHONE_MIN_LENGTH; ?>, "<?php echo ENTRY_TELEPHONE_NUMBER_ERROR; ?>");'
b. Under this entry insert
<!-- STUDENT INFO HERE -->
check_input("studentname", <?php echo ENTRY_FIRST_NAME_MIN_LENGTH; ?>, "<?php echo ENTRY_STUDENTNAME_ERROR; ?>");
<!-- END -->

4. /includes/template/YOURTEMPLATE/template/
a. Update ENTRY_???_TEXT fields that you hade previously entered for taxid info

<br class="clearBoth" />
<label class="inputLabel" for="studentname"><?php echo 'Students Name:'; ?></label>
<?php echo zen_draw_input_field('studentname', $account->fields['customers_studentname'], 'id="studentname"') . (zen_not_null(ENTRY_STUDENTNAME_TEXT) ? '<span class="alert">' . ENTRY_STUDENTNAME_TEXT . '</span>': ''); ?>