I want to modify some core behavior and introduce more features of postings and comments, such as defining multiple user circles and user types in flexible ways, disabling/enabling comments, advanced control over visibility of postings/comments with regards to different user circles, defining custom actions triggered by postings/comments, and other advanced behaviors that are not part of the core features offered out of the box.

So, I would obviously be going beyond plug-in development and want to change some foundational system features (note: not planning to contribute back to the core just yet, want to experiment on my own copy). Can you please advise me - how well is the core model documented and capable of being modified? How modular and loosely-coupled is the system design to allow modifications that don’t easily break the entire system? I want to know if this would be overall practical and not prohibitively difficult to accomplish with this software, and if others have gone in the same direction before me.