To give it a bit more legal bite with respect to ruling out accidental infringement, I suppose the passphrase could include a statement that assures that the sending program is licensed under the GPL.
Another thing is that if it is considered possible that the passphrase an be distributed as a separate entity and merely aggregated with the plugin, it could be licensed under a more stringent license than the GPL which is more effective at attaining the goal of requiring GPLed plugins. The plugin interface could compute a cryptographic hash which the license can then allow to be licensed under the GPL so it can be included into cc1 binary.
