Overwriting StringManipulator's Parse() method

I've been trying to overwrite the StringManipulator's Parse() method using an Extension, with the aim to render smilies after all the parsing has happened.
This doesn't work:
This doesn't work:
$Context->ObjectFactory->SetReference("StringManipulator", "RenderEmo");
class RenderEmo extends StringManipulator {
function Parse($String, $Object, $Format, $FormatPurpose) {
// this code never gets executed it seems
print 'hello.';
}
}
0
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Basically something to parse all the text of a post when it gets displayed, optionally with the ability to check for the type of text (Text, HTML, Markdown, etc.)