As KevinH pointed out, you really have to go through the OPF specs in detail to know what is supported and what isn't... and believe you me, I've spent so much time going over the OPF spec for epub 2 (which is what I'm still playing around with) that I'm seeing it even when I close my eyes!
So the properties I mentioned are part of the DC subset recommended by the IDPF... and hence the confusion.
I barely know what I'm doing. How am I supposed to teach 3 new boys how to make ebooks?
If one of them asks me why the IDPF has defined properties which no one uses or knows about, what am I supposed to say?
Oh, at least I've managed to confirm that I've got the syntax for the dc:date property correct.