That's because you have no clue how it works.

Posts are rendered into "real" html at page display time by the Engine, which expects the node source content to be in our special su{b|per}set of html.

We now have well over one million nodes in this format in the database.

No transformation is done to user-posted content at input time.

Ponder that for a while. Then please provide your ideas. The challenge is not insurmountable, but one must begin from an informed position.