Question

I want to add a custom message once someone saves a specific content-type telling them that it is going through an approval process. This will let them know as well as prevent them from re-submitting.

Answer 1

Your best bet is a small custom module that does something like this (completely untested):

<?php
function MODULENAME_nodeapi(&$node, $op, $a3 = NULL, $a4 = NULL) {
  if ($node->type == 'custom_node_type' && $op == 'insert') {
    drupal_set_message(t('Put your custom message here.'));
  }
}

For more info, see the docs for hook_nodeapi and drupal_set_message

Edited to reflect the below comments.