Until somebody writes a custom maven2 plugin for the openjpa enhancer, a simple way to do this is to invoke an ant script from maven like this:
<build>
<plugins>
<plugin>
<artifactId>maven-antrun-plugin</artifactId>
<executions>
<execution>
<phase>process-classes</phase>
<configuration>
<tasks>
<java classname="org.apache.openjpa.enhance.PCEnhancer"
classpathref="maven.runtime.classpath"
dir="target/classes" fork="true" />
</tasks>
</configuration>
<goals>
<goal>run</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</build>