How do I make the maven jetty plugin aware of an additional web directory?

Looks like this could help you:

• http://docs.codehaus.org/display/JETTY/Multiple+WebApp+Source+Directory

So i'd amend your configuration as follows:

<plugin>
  <groupId>org.mortbay.jetty</groupId>
  <artifactId>maven-jetty-plugin</artifactId>
  <version>6.1.12</version>
  <configuration>
    <webAppConfig>
      <contextPath>/${project.artifactId}</contextPath>
      <!-- Javascript files are not java class files, so you can skip this
      <extraClasspath>target/classes/:src/main/javascript</extraClasspath>
      -->
      <baseResource implementation="org.mortbay.resource.ResourceCollection">
        <resourcesAsCSV>src/main/webapp,src/main/javascript</resourcesAsCSV>
      </baseResource>
    </webAppConfig> 
  </configuration>
</plugin>

Note that the ResourceCollection class has moved in the latest version of Jetty (9.3.0.M2).

Therefore, the implementation should point at org.eclipse.jetty.util.resource:

<plugin>
  <groupId>org.eclipse.jetty</groupId>
  <artifactId>maven-jetty-plugin</artifactId>
  <version>9.3.0.M2</version>
  <configuration>
    <webAppConfig>
      <baseResource implementation="org.eclipse.jetty.util.resource.ResourceCollection">
        <resourcesAsCSV>src/main/webapp,src/main/javascript</resourcesAsCSV>
      </baseResource>
    </webAppConfig> 
  </configuration>
</plugin>