Custom textfield element, different $form_state value and element '#value' value

I have a textfield element (named 'bus_station'), which holds a bus station name.

I am looking for a way to create a custom 'bus_station' element (possibly extending Textfield class), which would:

• Still expect a text input in form of bus station name (and retain it during $form_state->setRebuild() and show it to user), but
• In $form_value, after form submit, either provide a FALSE (custom code in Element definition doesn't recognize input as a station) or TRUE (it does recognize a station)? Or Entity ID (ID of a matched bus station) and/or NULL (no match)?

This can be done via Form API callback implementation:

Inside module/src/Element/ElementClass.php, in public function getInfo() {} define a callback, e.g. processMyElement under '#process' key:

public function getInfo() {
  $class = static::class;
  return [
    // [...] Some other definitions.
    '#process' => [
      [$class, 'processMyElement'],
    ],
  ];
}

And then perform two complementary operations.

(1) Process the user input and return a value which will be provided in the '#form_state' values in valueCallback() method:

public static function valueCallback(&$element, $input, FormStateInterface $form_state) {
  $output = parent::valueCallback($element, $input, $form_state);
  if ($input == 'SOMETHING') {
    $output = 'SOMETHING_ELSE';
  }
  return $output;
}

(2) And vice versa, in the processMyElement() method return the original value:

public static function processMyElement(&$element, FormStateInterface $form_state, &$complete_form) {
  if ($element['#value'] == 'SOMETHING_ELSE') {
    $element['#value'] = 'SOMETHING';
  }
  return $element;
}

Usage example

I have used this to extend Drupal\Core\Render\Element\Textfield class in combination with JS Storage Complete. That gives me Autocomplete (without need for a callback URL). This autocomplete returns Entity ID obtained via entity load based on unique label inside valueCallback() method, but user is still presented with its text input (obtained by returning $entity->label() inside processMyElement() method).