class FooAdmin(ModelAdmin):
    …
    def lookup_allowed(self, key, value):
        # NOTE: Django 1.2.5 changed the call signature to add the value
        # Django 1.2.4 restricted the list of allowed lookups to only those
        # specified in list_filter or date_hierarchy, which doesn't help when
        # we need to filter on a list with thousands of options. We'll 
        # override that to allow the few which we actually use:
        if key in ('related__pk', 'related__custom_field'):
            return True

        return super(FooAdmin, self).lookup_allowed(key, value)