xsd "semantic" restriction based on both predefined values and values from the XML document

Question

Is it possible in XSD to define an enumeration restriction based also on content of the XML document?

e.g. I want this XML to be valid:

<object>
    <command name="add">
        <param name="layer" type="int" />
        <param name="position" type="point" />
    </command>
    <struct name="point">
        <param name="x" type="float" />
        <param name="y" type="float" />
    </struct>
</object>

according to this schema:

<?xml version="1.0"?>
<xsd:schema
    xmlns:xsd="http://www.w3.org/2001/XMLSchema"
    elementFormDefault="unqualified">

    <xsd:simpleType name="paramType">
        <xsd:restriction base="xsd:string">
            <xsd:enumeration value="int" />
            <xsd:enumeration value="float" />
            <xsd:enumeration value="double" />
            <xsd:enumeration value="bool" />
            <xsd:enumeration value="string" />
            <xsd:enumeration value="table" />
            <!-- XXX: include values from /object/struct/@name here??? -->
        </xsd:restriction>
    </xsd:simpleType>

    <xsd:complexType name="paramSpec">
        <xsd:attribute name="name" type="xsd:string" use="required" />
        <xsd:attribute name="type" type="paramType" use="required" />
    </xsd:complexType>

    <xsd:complexType name="objectSpec">
        <xsd:sequence>
            <xsd:choice maxOccurs="unbounded">
                <xsd:element name="command">
                    <xsd:complexType>
                        <xsd:sequence>
                            <xsd:element name="param" type="paramSpec" maxOccurs="unbounded" />
                        </xsd:sequence>
                        <xsd:attribute name="name" type="xsd:string" use="required" />
                    </xsd:complexType>
                </xsd:element>
                <xsd:element name="struct">
                    <xsd:complexType>
                        <xsd:sequence>
                            <xsd:element name="param" type="paramSpec" maxOccurs="unbounded" />
                        </xsd:sequence>
                        <xsd:attribute name="name" type="xsd:string" use="required" />
                    </xsd:complexType>
                </xsd:element>
            </xsd:choice>
        </xsd:sequence>
    </xsd:complexType>

    <xsd:element name="object" type="objectSpec" />
</xsd:schema>

because the value of the type attribute is either one of the basic types defined in paramType, or the name of a <struct> element defined in the XML itself.

It seems that <xsd:key>/<xsd:keyref> would do validation on the values from XML only. How to combine those with additional values?