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 Phorge source code?
(note that T4213 is "whishlist", and not fully implemented).
In most cases of "adding an implementation of something", the answer is just "implement the abstract class" - most things of them have a "getAll...()" static method that uses the class-loader to find all implementations. Sometimes there's a "priority" or "order" feature as well.
To add code, you can either dump it in /src/extensions, or add a library - see https://we.phorge.it/book/contrib/article/adding_new_classes/ for details.