Webform save draft after each field update
Ok, I've found this http://drupal.org/node/1319926 and based on it I've managed to make it work. This is my ajax callback (it can be attached using hook_form_alter(), based on your needs):

function _my_module_js_draft($form, &amp;$form_state) {
  global $user;
  $draft_sid = _webform_fetch_draft_sid($form['#node']-&gt;nid, $user-&gt;uid);

  $form_state['values']['details']['sid'] = $draft_sid;
  $form_state['values']['op'] = !empty($form['actions']['draft']['#value']) ? $form['actions']['draft']['#value'] : t('Save Draft');

  $form_state['save_draft'] = TRUE;
  webform_client_form_pages($form, $form_state);
  webform_client_form_submit($form, $form_state);

  return $form;
}


For having the save draft functionality available for Anonymous I've used this patch: http://drupal.org/node/1932560#comment-7185674