Users setting custom fields

I would like to allow users to set the custom fields when only they file a
trouble ticket. Thereafter, only members of the Helpdesk group should be
allowed to alter the custom fields (eg, the user misidentified his
workstation’s hardware). My Helpdesk queue follows the Yoyodyne example
from the “RT Essentials” book. Other permissions set are
"AssignCustomFields" and “SeeCustomFields” for unprivileged users (aka
users who’s only interaction with the ticket system is for the helpdesk).
These permission flags do not appear selectable on a queue-by-queue basis,
which is what I think I really want (is that the wrong idea?). Anyhow,
despite these settings, unprivileged users are not allowed to set the
custom fields. Would someone please help me?

David Griffith