How to specify a mixed table-per-subclass -> table-per-hierarchy in NHibernate HBM?

Question

I'm mixing table mapping strategies to store a class hierarchy via NHibernate 2.1.2. I'm not using Fluent NHibernate.

I understand how to map a table-per-hierarchy -> table-per-subclass structure, but not the other way around, as the XML is invalid after adding the discriminator.

Here's the HBM extract:

  <class name="Base1" table="Base1">
    <id name="Id" column="Id" type="int" />
    <property name="Name" type="string" not-null="true" />
    <property name="Description" type="string" not-null="false" />

    <joined-subclass name="Base2" table="Base2">
      <key column="BaseId" />
      <set name="Children">
        <key column="BaseId" />
        <one-to-many class="Base2" />
      </set>
      <many-to-one name="Parent"
                   class="Base2"
                   column="ParentId"/>
      <joined-subclass name="Base3" table="Base3" discriminator-value="-1" abstract="true">
          <key column="BaseId" />
          <discriminator column="ClassType" type="int" />
          <many-to-one name="Foo"
                       class="Foo"
                       column="FooId"/>
          <subclass name="RedClass" discriminator-value="1" />
          <subclass name="BlueClass" discriminator-value="2" />
        </joined-subclass>
    </joined-subclass>
  </class>

The problem is with Base3 and subclasses, since joined-subclass doesn't permit the discriminator-value attribute or the discriminator child element.

I'd like to keep this structure since it's a good trade-off between storage efficiency and flexibility for the base of the hierarchy.