Is it possible to extend twig with 'Twig_SimpleTest'?
When you call Craft's TemplatesService->getTwig() (craft()-&gt;templates-&gt;getTwig()), you get an instance of the Twig environment and from there you could be able to call addTest.

100% untested, but something like this should work:

$twig = craft()-&gt;templates-&gt;getTwig();
$twig-&gt;addTest(new My_Twig_SimpleTest());