I realise that the first thought is "ah, it won't load" but I have countered that by ensuring it loads as I have a k2 content module in the footer of every page loading the latest video. Let me explain what I have done and see if anybody has a simple answer.
I have added the following code to my tag.php override.
<div class="tagItemView">
<?php
$dispatcher = JDispatcher::getInstance();
JPluginHelper::importPlugin('content');
$item->text = $item->video;
$item->video = $item->text;
?>
<div class="tagItemVideo">
<?php echo $item->video; ?>
</div>
and this is working to print the plugin tag as follows
{youtube}_YcCLCy4V98{/youtube}
Even though
<link rel="stylesheet" href="/trod13/plugins/content/jw_allvideos/jw_allvideos/tmpl/Responsive/css/template.css" type="text/css" />
<script src="/trod13/plugins/content/jw_allvideos/jw_allvideos/includes/js/behaviour.js?v=4.5.0" type="text/javascript"></script>
<script src="/trod13/plugins/content/jw_allvideos/jw_allvideos/includes/js/mediaplayer/jwplayer.min.js?v=4.5.0" type="text/javascript"></script>
<script src="/trod13/plugins/content/jw_allvideos/jw_allvideos/includes/js/wmvplayer/silverlight.js?v=4.5.0" type="text/javascript"></script>
<script src="/trod13/plugins/content/jw_allvideos/jw_allvideos/includes/js/wmvplayer/wmvplayer.js?v=4.5.0" type="text/javascript"></script>
<script src="/trod13/plugins/content/jw_allvideos/jw_allvideos/includes/js/quicktimeplayer/AC_QuickTime.js?v=4.5.0" type="text/javascript"></script>
are loading in my head tags due to the, correctly functioning, video in the footer.
So, how do I enable K2 content plugins in my tag.php file?