Concatenation with %keyword in mini-panels
I've figured out but not sure either it's a standard way or not but I believe override title field is not accepted "prefix or postfix" for %keyword like mentioned in the question.

Solution

1) Removed tag h1 from the attached image in the question.

2) Created new tpl named as panels-pane--event-node-title.tpl.php and added some code in it

&lt;div class="pane-content"&gt;
  &lt;?php if ($title): ?&gt;
  &lt;h1&gt; &lt;?php print $title; ?&gt; &lt;/h1&gt;
  &lt;?php else : ?&gt;
    // Here we can set any prefix or postfix within the tag
    &lt;h1&gt; Event in &lt;?php print render($content); ?&gt; &lt;/h1&gt;
  &lt;?php endif; ?&gt;
&lt;/div&gt;


$content variable will get content of the field along with its all settings (ID, class, tag etc.)

I hope this will help someone.