I see. After playing with it for some time I finally manually put the lib jar in a folder 'lib' in the ear. This worked, thanks!
Also, for anyone out there using Maven, this was my problem:
What I had:
<jarModule>
<groupId>someGroupName</groupId>
<artifactId>someArtifactName</artifactId>
<includeInApplicationXml>true</includeInApplicationXml>
</jarModule>
What I have now that works:
<jarModule>
<groupId>someGroupName</groupId>
<artifactId>someArtifactName</artifactId>
<bundleDir>lib</bundleDir>
</jarModule>
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