Since your schema can be combined with other schemas at run-time, we are
not allowed to map that IDREF to Carrier by default.
But I think there's an unsupported customization that lets you do this,
if you are willing to go there.
I'm writing this e-mail from a different machine I usually use, so I can
give you the detailed information right now, but stay tuned...
--
Kohsuke Kawaguchi <kohsuke.kawaguchi_at_sun.com>