Override default value for attribute in base type

I have an existing type like this, which belongs to a library-level XSD:

    <xs:complexType name="mybase">
      <xs:attribute name="myattr" type="xs:string" default="123"/>
    </xs:complexType>

And later this is extended by a number of more specific XSD's that implement application specific configuration requirements:

    <xs:element name="appconfig">
      <xs:complexType>
        <xs:complexContent>
          <xs:extension base="mybase">
            <xs:attribute name="specific_attr" type="xs:string" use="required"/>
            <!-- etc, more specific requirements -->
          </xs:extension>
        </xs:complexContent>
      </xs:complexType>
    </xs:element>

We use this all over the place, but have just run across a use case where for a specific application we want to use a different default value for "myattr". I've poked around various XSD documentation, but haven't found anything that seems to be able to do this, is it possible?