I solved this by changing the expression to:
reverse( 'django-admin', args=["%s/%s/%s/" % (app_label, model_name, object_id)] )
This requires/assumes that the root url conf has a name for the "admin" url handler, mainly that name is "django-admin",
i.e. in the root url conf:
url(r'^admin/(.*)', admin.site.root, name='django-admin'),
It seems to be working, but I'm not sure of its cleanness.