XMI12 — Problem with namespaces

In any metamodel it is possible to specify an xmi namespace for a package using xmi.namespace tag, however the tag value seems to contain only the name of the namespace. How is the XMI writer supposed to know the URI of the namespace to be able to generate the namespace declaration into the XMI file?

Concrete example:
MOF has an xmi.namespace=Model tag attached to the Model package.
Now suppose I have a MOF metamodel and want to serialize it into the XMI. The XMI writer needs to declare Model namespace like this:
<XMI xmlns:Model="org.omg.mof.Model">
Where is the XMI writer suppose to get the "org.omg.mof.Model" thing to generate it into the XMI? It is not explicitly specified anywhere in the MOF 1.4 model. Or can I generate just any URI I like?