"Client module dependencies" don't seem to work

If you just want more than one dependency whenever someone references libs.jdom, then you don’t need client modules.

You can simply do:

ext.libs = [
  jdom : [
      'org.jdom:jdom:1.0',
      'org.jdom:jaxen-jdom:1.0-FCS',
      'org.jdom:jaxen-core:1.0-FCS'
    ]
  }
]

It’s actually quite a useful mechanism: Optional means: “I also integrate with this other library if you use it. If not, that’s okay”.