Cat III autolands on non-Cat III approaches
Disregarding the regulatory and procedural restrictions, is it technically possible to do an autoland on an ILS approach that is not certified for autoland? Will the on-board automation catch this somehow and refuse to do it, or will it autoland on any ILS to which it is pointed?
If the latter is the case, do pilots ever autoland for currency in clear weather on an approach that isn't certified for it, or do they always wait until they actually have a Cat III approach to use?
Also, if anyone knows what technically distinguishes a Cat III ILS from a lower level ILS, I'd be interested in knowing about it. Are the tolerances tighter for Cat III approaches, or are there other, qualitative differences in the way the electronics and radio signals work? From what I've read, I get the impression that the only thing that makes a Cat III a Cat III is that it has been tested and certified to provide a specific accuracy, whereas other more ordinary ILS approaches have simply not be tested (even though they might still qualify in terms of accuracy).