How to pass additional arguments in $form['#after_build']
Most of callbacks like #after_build will received automatically 2 arguments: $elements and $form_state, like this : 

my_callback_after_build($element, &amp;$form_state){
//...
} 


So you simply have to push your var in the form_state var from where you add your callback : 

function custom_third_level_auto_suggest_field_widget_form(&amp;$form, &amp;$form_state, $field, $instance, $langcode, $items, $delta, $element) {
   $form['#after_build'] = 'custom_third_level_auto_suggest_widget_selection_afterbuild';
   $form_state['foo'] = 'bar';
}


And you'll get it in the form_state var of your callback like this: 

function custom_third_level_auto_suggest_widget_selection_afterbuild($form, &amp;$form_state){
   var_dump($form_state['foo']); //will print "bar"
}