form.html

<form action="save.php" method="POST">
  <input type="hidden" name="element-uid" value="[tt_content_uid]" />
  <textarea>Here goes comment - this could be a RTE</textarea>
</form>

gistfile1.txt

<a href="#" onclick="openNoteForm">Open note form</a>

<script>
function openNoteForm(element) {
  jQuery('#formid').show() <<-- here, do some magic that makes sense: Show the form (form.html) set the value of the hidden field to the current tt_content element
}
</script>

save.php

<?php

$sql = 'INSERT INTO `table` SET note = "' . $notecontent . '" element = "' . $tt_content_uid . '" fe_user = "' . $this->getCurrentUserSession() . '"';

functon getCurretUserSession() {
  return $GLOBALS['TSFE']->fe_user # and maybe some more.. 
}

?>