I've tried changing the /etc/glance/policy.json and setting the following:
"publicize_image": "role:member",
And then restarting both the glance-api and glance-registry services but of course that would be too easy so it doesn't work :)
openstack image create --file gammy_image.iso JACOBS-GAMMY-IMAGE --public
403 Forbidden
You are not authorized to complete publicize_image action.
(HTTP 403)