Ah, note that reactor-resolution only works for classes and resources (project.build.outputDirectory), and specifically not for web resources. Those would have to be computed out of the MavenProject#getProjectReferences, if at all possible (the Tomcat plugin doesn't do it, see MTOMCAT-158). A workaround would be to use mvn -pl my-webapp -am package jetty:run but then Artifact#getFile would return the packaged JAR for the dependencies, defeating the purpose of launching jetty:run in a reactor build (which is to use the build.outputDirectory directly, so the webapp can be restarted easily while your IDE compiles the classes there on-the-fly, and without the need to package the dependencies). Maybe for overlays support, the plugin could require that prepare-package or war:exploded by run previously on the WAR dependencies, and then look at the output for these (<project.build.directory>/<project.build.finalName>) out of the getProjectReferences.
See also MNG-5214: for now (pending Maven 3.1.0), Maven will give you the build.outputDirectory for a dependency with type war; after the fix, Artifact#getFile() will instead return null.