JAXB Custom Bindings to generate Constants

Question

I'm generating Java classes from a xsd file provided by a 3rd party (the ONIX XML file XSD to be precise). The schema defines quite a few types, that have a shortname and a refname attribute. Unfortunately, the ONIX guys did not set the single allowed string value for those fields, using the fixed attribute, but instead they did it like this:

<xs:attribute name="refname">
    <xs:simpleType>
        <xs:restriction base="xs:token">
            <xs:enumeration value="AddresseeIDType"/>
        </xs:restriction>
    </xs:simpleType>
</xs:attribute>

<xs:attribute name="shortname">
    <xs:simpleType>
        <xs:restriction base="xs:token">
            <xs:enumeration value="m380"/>
        </xs:restriction>
    </xs:simpleType>
</xs:attribute>

This causes JAXB to generate mutable String field's for the shortname and refname property. The information about the only valid values for those fields is lost during the class generation process.

Is there any way to use an external JAXB bindings file to have XJC generate the fields for shortname and refname in the following form:

@XmlAttribute(name = "refname")
protected static final String refname = "AddresseeIDType";

@XmlAttribute(name = "shortname")
protected static final String shortname = "m380";

I know there is a fixedAttributeAsConstantProperty attribute, but that does only seem to work, if the xsd would instead define the shortname and refname in the following manner:

<xs:attribute name="refname" fixed="AddresseeIDType" />
<xs:attribute name="shortname" fixed="m380" />

Any help appreciated