Adamw, to be honest this RFC already feels a little tainted. It started with a set endpoint in mind without debating whether the rationale for that end was there. The rationales for such an idea were never explained in proper detail. And even if we start expanding it at this point it's going to remain confusing and the way it started will probably continue to act against whatever you really want.
I suggest you start a new RFC. Instead of suggesting extensionification, componentization, etc... right from the start; start listing different ideas for improvements to the protection system. ie: "A fully controllable ACL system; With the ability to control rights per user. And specific conditionals like age and how many edits instead of just the autopromote settings." Then we can start discussing things like "Should this be part of core, or an extension?", "If it's in core, how do we handle the old system? Do we keep them both around working mutually? Or it it possible to rewrite the old system as a subset of the new system that just uses a different UI.", and "If it's an extension. What hooks, interfaces, and changes are we missing to make it possible to write? (A question that often requires actually trying to write the code and attempts to use the existing hooks first.)".