I'd disagree Dave. I agree that the Captain does have absolute authority to
deny entry (even to those on an "authorised" list), but I'd argue that his authority cannot override his national law. So in effect, he can't "add" to an authorised list, as that list is set by legislation. That's certainly how my company sees it.
it would be hard to argue that anyone legitimately invited to the flight deck by him/her isn't then duly authorised to be there
Not so, the UK SCD lays out in black and white which categories of person can be authorised by a company. As I alluded to earlier, it doesn't include "anyone invited by the Captain".