I'd like to be able to pass a custom producer the path to a configuration file. If I implement Configurable would that do the trick? What would I have to add to the Cocoon.properties file in order to pass at least a property to my custom producer?

Yes it did.

/* Add producer.myProducer.property in Cocoon.properties 
** The preceding producer get's it sent along to the producer factory
** make myProducer implement Configurable and give it an init method.
*/

public void init(Configurations conf) {
        this.myProperty = (String) conf.get("myProducer.property");
}