In the documentation codec is marked as one of the plugins options. The expected behavior is that the codec is used as the message body (eg. codec => plain, should give the message as is). However json is always the returned format.
This seems to be confirmed by line 86 of the plugin's code :
t.send(event.sprintf(@destination), event.to_json, headers)
Aka event.to_json seems to short circuit the codec.