Extensible Enumerations Technique

I know techniques for extensible enumerations in W3C XML Schema 1.0 have 
been discussed on this list before (extensively), but I didn't find the 
exact technique below, so I thought I'd post it here to get 
feedback/comment.

Enumeration types (say, Enumerated_Type) are defined as xs:union between 
a Enumerated_Type_Base and Enumerated_Type_Ext in a base XSD file:

<xs:simpleType name="Enumerated_Type">
   <xs:union memberTypes="Enumerated_Type_Base Enumerated_Type_Ext"/>
</xs:simpleType>

<xs:simpleType name="Enumerated_Type_Base">
   <xs:restriction base="xs:NMTOKEN">
      <xs:enumeration value="ExistingValue"/>
      ...
   </xs:restriction>
</xs:simpleType>

<xs:simpleType name="Enumerated_Type_Ext">
   <xs:restriction base="xs:NMTOKEN"/>
</xs:simpleType>

In the xs:redefine section of a separate extension XSD file redefines 
the Enumerated_Type_Ext:

<xs:simpleType name="Enumerated_Type_Ext">
   <xs:restriction base="Enumerated_Type_Ext">
      <xs:enumeration value="ExtendedValue"/>
   </xs:restriction>
</xs:simpleType>

XSLT stylesheets are then optionally used to remove xs:redefine and 
xs:union given that many tools (particularly XML/OO language binders) do 
not support these constructs.

Thanks in advance,

Guillaume