Why is hook_form_alter() not setting default values in _after_build function on /node/add/myct?
After doing some research, it turns out that taking a JavaScript/jQuery approach is the wrong way to do this. Rather, the code should use hook_form_alter() to retrieve POSTed data ($form['#method'] = 'post';) and set the default values of the node form (including CCK fields) via a function specified with $form['#after_build'].

So, if you want to dig in here and start monkeying with the "real" properties, you need to come in after the point that the CCK form elements are processed. Luckily, the form API has a phase designed for exactly this purpose: #after_build.

/**
 * Implements hook_form_alter().
 */
function example_form_alter(&amp;$form, $form_state, $form_id) {
  // Check for a particular content type's node form.
  if ($form_id == 'example_node_form') {
    // Add an after_build function to process when everything's complete.
    $form['#after_build'][] = 'example_after_build';
  }
}

/**
 * Modifies CCK form elements on the example node form.
 */
function example_after_build($form, &amp;$form_state) {
  // Your code here.
  return $form;
}