pom.xml

          <plugin>
            <groupId>org.codehaus.gmaven</groupId>
            <artifactId>gmaven-plugin</artifactId>
            <version>1.5</version>
            <executions>
              <execution>
                <phase>initialize</phase>
                <goals>
                  <goal>execute</goal>
                </goals>
                <configuration>
                  <source>
                    /* If required
                    System.setProperty('http.proxyHost', host)
                    System.setProperty('http.proxyPort', port)
                    */
                    def tmpDir = System.getProperty('java.io.tmpdir')
                    File file = new File(tmpDir, 'graphviz-2.38.zip')

                    def ant = new AntBuilder()
                    if (!file.exists())
                      ant.get(src: "http://www.graphviz.org/pub/graphviz/stable/windows/${file.name}", dest: file)
                    if (!new File(tmpDir, 'release').exists())
                      ant.unzip(src: file.absolutePath, dest: tmpDir, overwrite: true)
                  </source>
                </configuration>
              </execution>
            </executions>
          </plugin>

Just setup other plugins to use ${java.io.tmpdir}/release/bin/dot.exe as dot executable.