So, what is the difference between a CATII autoland and one at CATIII minima, the autoland part itself should be the same?
Tom my understanding is that an autoland is optional at Cat II or higher. You can do an autoland any time with an ILS though manufacturers and airlines will have specific guidelines on the subject. There are risks associated with the protected areas which are not guaranteed except in low vis conditions. ( Some operators have a specific ban on autolands at Cork regardless of the weather - usually after an event )
I read about the metal plate mentioned with Zurich as an example. It might work ( that would be beyond my expertise ) but Cork has two important differences to Zurich. Zurich is set low between a number of hills so a rapid incline up to the threshold doesn't confuse the rad alt, unlike Cork and Zurich doesn't have any significant hump at the 1000' point which causes the autoland to flare too high and subsequently float.
An autoland is mandatory for ( almost all - HUD displays may be different ) Cat III operations.