How to run custom tasks with Herald Rules?

For now I have a ugly idea: Change content in PhabricatorMailImplementationPHPMailerAdapter.php and add some lines to invoke another python class of my own.

We have lots of APIs for external things, but without knowing what you're trying to do specifically, we can't really give you advice here. Herald / Mail are absolutely never the correct answer. Doorkeeper, Harbormaster, Feed Hooks, all available out of the box.