Thanks again for the replies. I've been investigating the BIOS options you both mentioned. I've tried 3 different BIOS versions (starting with default settings) and have not managed to get VT-d working, with settings as expected or reversed.
I installed MS Windows in order to use the HP Performance Advisor Software. It confirms I have the updated motherboard (0B54h) which tallys with the BIOS block date, and from the HP Specs in my intial post, this SHOULD mean that I have the C2 stepping.
However, it turns out that Xen identifies the 5520 chipset as faulty on boot and disables Interrupt Remapping. I also confirmed in my dmesg log after booting Xen, that indeed, I appear to have the 13h version, which is the B3 stepping.
I rather suspect that's the end of the line as far as using this Z600 for virtualisation. This 55x0 chipset issue seems to affect the whole range of Z400/600/800 machines as far as I've read, and I can't find any evidence at all that anybody has had success with Vt-d on any of these machines.
I've got QubesOS v4 working with PV instead of Vt-d, but that wasn't really the goal.
Good luck to anybody who stumbles upon this thread following the same expectiations I had!