[SMARTY] SearchEngineSafeURL plugin?

I've written a plugin that rewrites hrefs to be search engine friendly.
and a piece of code that has to be run at the beginning of the php
script to unbundled the url into variables.  I have the output filter
working correctly.  but can't find how to register the function that
build the variables.  is there a hook to allow php code to be executed
near the beginning say when the Smarty Class in instantiated.
new->smarty()?