A BBCode thing for IPB, take a list of comma separated values as the option and only show the content in those topics. For example "[showin=123,456]Hello![/showin]" would only show in topics of the ID 123 and 456.

showin.html

<div class="showintopic">
{content}
</div>
<script type="text/javascript">
  var showintopics = "{option}";
  var topic = String(ipb.topic.topic_id);
  var topics = showintopics.split(",");
  
  if (jQuery.inArray(topic, topics) != '-1') {
  	jQuery('script:last').prev('.showintopic').show();
  }
</script>

style.css

.showintopic {
	display: none;
}