Added this to Common.xsd:
    <xsd:simpleType name="Output">
        <xsd:restriction base="xsd:string">
            <xsd:pattern value="(.*)"/>
        </xsd:restriction>
    </xsd:simpleType>

Then I changed the "type" for OutputField in Rules.xsd to "sxi:Output", like so:
    <xsd:element name="OutputField" type="sxi:Output" minOccurs="1" maxOccurs="1" default="Omit"/>

But the following line is still unhappy in my CONTRACT1.xml config:
    <sxi:OutputField>12</sxi:OutputField>