Thanks for your answer.
I think what I want to do is to improve jbpm3 without changing the source code(http://code.google.com/p/jbpm-guice/). So to speak I tried to use the process.class.loader.factory to actually replace the jbpm class instantiation mechanism without the need to change the core source code. However, my initial question and thus my assumption that it is not possible to upload jar's to the meaven central repo that have transitive dependencies that are not in the maven central repo (e.g. jbpm3-jpdl) was wrong. It seems to be okay to add the jboss repo to your artefact. I could upload my artifact to the maven central repo and have now the dependency
<dependency>
<groupId>com.google.code.jbpm-guice</groupId>
<artifactId>jbpm3-guice</artifactId>
<version>2.0.2</version>
</dependency>