DokuWiki_Syntax_Plugin in lib/plugins/syntax.php doesn't extend DokuWiki_Plugin, while other plugin types do.
But the most stuff is already implemented in DokuWiki_Syntax_Plugin
An exception is loadHelper($name, $msg)
Why is this one not implemented? But is the advice  to do:
$tag =& plugin_load('helper', 'tag');
$entries = $tag->tagRefine($entries, $refine);
This is same as what loadHelper does.
Proposal: implement also loadHelper in DokuWiki_Syntax_Plugin
Is this ok for you?