This is a reasonably fundamental issue of is the plugin should be separate or extend AuthComponent (Using the startup() method is replicating the behaviour of AuthComponent).

Some methods have to be provided via a component (e.g. the OAuth class methods) and so the main argument for separation is that it could all be done in a single component. So that's why I chose not to build it as Authentication object.

If it could be shown to work nicely then I would probably be in favour of switching to that pattern of a component + an authentication object, and I don't see any reason to offer the old pattern as an option?
Is this towards what you were thinking?