Setting featured image based on related post

I took a quick look at the Modern Tribe’s source code, particularly the single-event.php view template. MT is using an internal function, tribe_event_featured_image(), which includes a filter: tribe_event_featured_image.

You could use this filter to see if MT has already found an image, and, if not, look up your own and return its value.

add_filter( 'tribe_event_featured_image', 'wpse174412', 10, 4 );
function wpse174412( $featured_image, $post_id, $size, $image_src ) {
  if ( ! $featured_image ) {
    // get image from related organizer
  }
  return $featured_image;
}