keyrefs from list type (ala IDREFS)

Is it possible to apply keyref constraints to each item in a list
attribute? For example:
 
<root>
    <item id="1" />
    <item id="2" />
    <aggregate items="1 2" />
</root>
....
<xsd:unique name="unique-ids">
    <xsd:selector xpath="./item" /> 
    <xsd:selector xpath="@id" />
</xsd>
....
<xsd:attribute name="items" use="required">
    <xsd:simpleType>
        <xsd:list itemType="xsd:nonNegativeInteger" />
    </xsd:simpleType>
</xsd:attribute>
 
In the above example, ID and IDREFS would do what I want, but in my
particular application, I need more granular control over the scope. Is
there a way to use the above schema and apply a constraint, ensuring
that the ids listed in items correspond to valid 'id's?
 
My apologies if this is a FAQ. I searched the list archive (and web),
and I could only find one question related to this (with no responses)
and a second response which maybe indicated that it's not possible.
 
Thanks in advance for any help (and sorry about the bloated corporate
..sig),

Gavin Kistner
Product Designer
Anark Corporation
1434 Spruce Street, Suite 200
Boulder, Colorado 80302 

[P] 303-545-2592, x256
[F] 303-545-2575
gavin.kistner@anark.com
http://www.anark.com <http://www.anark.com/> 
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