How do I write a custom Processor which sends multiple messages?

You could use a Splitter or use multiple Message Translator instances in your route.

Or you could write a custom processor which is injected with a ProducerTemplate instance that just generates N messages…

public class MyProducer implements Processor {
  ProducerTemplate producer;

  public void setProducer(ProducerTemplate producer) {
    this.producer = producer;
  }

  public void process(Exchange inExchange) {
    // some loop for each message
    for (String template in templates) {
       // lets send a new exchange to the producers default destination
       // being called back so we can customize the message
       producer.send(new Processor() {
          public void process(Exchange outExchange) {
              outExchange.getIn().setBody("This is the body");
              // set some headers too?
          }
       });
    }
}

Then the ProducerTemplate can be injected — configured in spring.xml with its default URI:

<camelContext xmlns="http://camel.apache.org/schema/spring">
  <template id="myTemplate" defaultEndpoint="activemq:someQueue"/>
</camelContext>

<bean id="foo" class="MyProducer">
  <property name="producer" ref="myTemplate"/>
</bean>

Note that the default output URI is inherited from the <template/> configuration. If you prefer you could specify that in the producer.send() method call.