In the build.xml of the project (likely this will be a Java Class Library project), override -post-compile or -pre-jar and invoke <openjpac/>. You will need to add the build/classes and the openjpa jars to <taskdef/> and <openjpac/>. The <openjpac/> will enhance all classes mentioned in persistence.xml (which has to be in the classpath)
<?xml version="1.0" [...]
