This post details on how I was able to add new template tag on Magento Cms pages and static blocks. If you want to know more about these directives and template tags, you can check official magento wiki here.
I have already added this functionality and example on my github repository here https://github.com/dbashyal/Magento-General-Module-Extension. This module consists of other functionalities, so check code and strip out that you don’t need it. http://dlvr.it/6KqnMd