Maybe it should be changed somewhere here in Display.template.php?
Can I use (copy) the part with if ($context['can_quote'] to add a quickbutton for reply?
// Can the user quick modify the contents of this post? Show the quick (inline) modify button.
if ($message['can_modify'])
echo '
<li class="listlevel1 quick_edit" id="modify_button_', $message['id'], '" style="display: none"><a class="linklevel1 quick_edit" onclick="oQuickModify.modifyMsg(\'', $message['id'], '\')">', $txt['quick_edit'], '</a></li>';
// Can they reply? Have they turned on quick reply?
if ($context['can_quote'] && !empty($options['display_quick_reply']))
echo '
<li class="listlevel1">
<a href="', $scripturl, '?action=post;quote=', $message['id'], ';topic=', $context['current_topic'], '.', $context['start'], ';last_msg=', $context['topic_last_message'], '" onclick="return oQuickReply.quote(', $message['id'], ');" class="linklevel1 quote_button">', $txt['quote'], '</a>
</li>';