Alter form to have single radio button term reference checked by default
the way I did it now, is following - First I tried to use a hook provided by workbench access itself which supposed to be for that use case, look here
https://drupal.org/node/2019473

But it would not work for me, so I did : 
this results in a select box with only one possibility. Not the best UX style maybe and later I will try to hide the element, maybe with ds module.

function workbench_access_crud_by_state_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id != 'hardware_antrag_node_form') {
    return;
  }
  //sdpm($form);
  $option_arr = $form['field_tu'][LANGUAGE_NONE]['#options'];
  // remove 'none'-option
  array_shift($option_arr);

  // look for field for workbench_access and replace options
  $wb_access_field = $form['workbench_access_fields']['#value'][0];
  $form[$wb_access_field][LANGUAGE_NONE]['#options'] = $option_arr;
}