How does one get JAXB-generated sources onto the Eclipse build path under m2e/Indigo

You have to tell M2E that it's okay to run your code generator as part of the Eclipse build:

<project>
  <build>
     [...]
     <pluginManagement>
      <plugins>
        <!--This plugin's configuration is used to store Eclipse m2e settings only. It has no influence 
          on the Maven build itself. -->
        <plugin>
          <groupId>org.eclipse.m2e</groupId>
          <artifactId>lifecycle-mapping</artifactId>
          <version>1.0.0</version>
          <configuration>
            <lifecycleMappingMetadata>
              <pluginExecutions>
                <pluginExecution>
                  <pluginExecutionFilter>
                    <groupId>org.codehaus.mojo</groupId>
                    <artifactId>exec-maven-plugin</artifactId>
                    <versionRange>[,)</versionRange>
                    <goals>
                      <goal>java</goal>
                    </goals>
                  </pluginExecutionFilter>
                  <action>
                    <execute/>
                  </action>
                </pluginExecution>
              </pluginExecutions>
            </lifecycleMappingMetadata>
          </configuration>
        </plugin>
      </plugins>
    </pluginManagement>
  </build>
</project>

Notes:

1. It's important to put this configuration in the pluginManagement section, not directly under plugins.
2. This will cause M2E to perform all java executions specified in your POM as part of every Eclipse build. This means they will run often and be a big nuisance if they are slow. I'm not sure how you would get M2E to run some of them and skip others. You'd probably have to place the unwanted executions in profiles.

You need to attach the source directory with the build-helper-plugin.

Like so:

 <plugin>
    <groupId>org.codehaus.mojo</groupId>
    <artifactId>build-helper-maven-plugin</artifactId>
    <executions>
        <execution>
            <id>add-source</id>
            <phase>generate-sources</phase>
            <goals>
                <goal>add-source</goal>
            </goals>
            <configuration>
                <sources>
                    <source>${project.build.directory}/generated-sources/java/</source>
                </sources>
            </configuration>
        </execution>
    </executions>
 </plugin>

You will also need to:

• Install the "Apt M2E Connector" from the Eclipse Marketplace. To do so click the error in the Overview tab of your pom.xml and select "Discover".
• Ensure there are no plugin execution filters for the build-helper-maven-plugin (see https://www.eclipse.org/m2e/documentation/m2e-execution-not-covered.html)