I found a reference to the CelerityPostprocessor: https://secure.phabricator.com/T4213
And i found the technical documentation: https://secure.phabricator.com/book/phabdev/article/celerity/
While digging in the code base i found several implementations of the CelerityPostprocessor.
And i probably found the location where the actual processor is attached to the Phorge instance: ./phabricator/src/__phutil_library_map__.php
What i am missing is, (how) can i attach my own CelerityPostprocessor to my local instance without modifying the source code?