Jesper,
I ended up adding a META-INF/jboss-deployment-structure.xml that included
<?xml version="1.0" encoding="UTF-8"?>
<jboss-deployment-structure xmlns="urn:jboss:deployment-structure:1.0">
<deployment>
<dependencies>
<module name="deployment.test.rar" slot="main" />
</dependencies>
</deployment>
</jboss-deployment-structure>
That seems to resolve the class loading issue.