How do I add an Implementation-Version value to a jar manifest using Maven?

You would use the Maven Archiver:

<plugins>
    <plugin>
      <groupId>org.apache.maven.plugins</groupId>
      <artifactId>maven-jar-plugin</artifactId>
      <configuration>
        <archive>
          <manifest>
            <addDefaultImplementationEntries>true</addDefaultImplementationEntries>
          </manifest>
        </archive>
      </configuration>
    </plugin>
  </plugins>

This will add the following to the manifest file:

Implementation-Title: ${pom.name}
Implementation-Version: ${pom.version}
Implementation-Vendor-Id: ${pom.groupId}
Implementation-Vendor: ${pom.organization.name}