Well, I have written a small extension that provides a simple API to store custom data in the ParserOutput object and in the QuickTemplate object. In order to make use of it, I would like to have a hook in SkinTemplate.php right before the QuickTemplate is executed. Therefore, I had to patch SkinTemplate.php for getting the hook. See http://wikivoyage.org/tech/CustomData-Extension for detailed explanation.
Hans