This is a workcentre 7556 btw. Turns out the firmware needed updating, the auto-register primary pin was a known issue. Upgraded to latest general release (061.121.222.06507). This fixed it.
I am trying to configure the auto-register primary pin feature inside xerox secure access. I cannot seem to work out how the various checkboxes should be set to allow this to function. We have independently tested and confirmed the external id and password option, and secure access pin only option.