Form element #value_callback: assembles the allowed values for 'boolean' fields.
function options_field_settings_form_value_boolean_allowed_values($element, $input, $form_state) {
$on = NestedArray::getValue($form_state['input'], $element['#on_parents']);
$off = NestedArray::getValue($form_state['input'], $element['#off_parents']);
return array(
$off,
$on,
);
}