Added this to the tracker: https://tracker.moodle.org/browse/MDL-56611
Interestingly - the only way you can achieve this is to either:
a) Edit moodle core (as per the tracker item), or
b) Utilise a non-utilised mdl_user field (like department, etc.) then change the language string to suit. Of course, while this will display for users ok, it does mean you are storing data about X in a table attribute called Y (which isn't great).
It would be much better if moodle core supported additional profile fields in the showuseridentity value - so that we can add additional profile fields that are fit for purpose.
Thanks Jason