Support polymorphic assocations

Consider this data model:

Container (1) ===> (n) BaseItem
                        ^    ^
                       /      \
                      /        \
                   FooItem   BarItem

such that the corresponding Java source would be

public class Container {
    private List<BaseItem> items;
}

public abstract class BaseItem {}

public class FooItem extends BaseItem {}

public class Bartem extends BaseItem {}

and a valid input document could be

<container>
  <items>
    <fooItem/>
    <barItem/>
    <barItem/>
    <fooItem/>
  </items>
</container>

i.e. the name of the XML element would be used to instantiate the proper subclass. The essential difference to Modello's current capabilities would be that different objects are collected in one list and the document order of the elements is retained.