I have an instance document where the same element has a wide variety
of Name and Value attributes:

<Data>
  <Info Product="XXX" Type=""/>
  <Detail Name="Added" Value="2009-01-01"/>
  <Detail Name="Cost" Value="25.99"/>
</Data>

There are 50-80 different Nave/Value pairs, all having very specific
validation needs.
I'd like to be able to do this for each pair:

<xsd:element name="Detail" id="Added">
    <xsd:complexType>
      <xsd:attribute name="Name">
        <xsd:simpleType>
          <xsd:restriction base="xsd:string">
            <xsd:enumeration value="Cost"/>
          </xsd:restriction>
        </xsd:simpleType>
      </xsd:attribute>

      <xsd:attribute name="Value">
        <xsd:simpleType>
          <xsd:restriction base="xsd:string">
            <xsd:pattern value="^\d{2}\.\d{2}$"/>
          </xsd:restriction>
        </xsd:simpleType>
      </xsd:attribute>
    </xsd:complexType>
  </xsd:element>

But, since each rule is based on the Detail element, it is illegal.

Is it possible to create a XML Schema for this, without having to use
XSI in the instance doc?

Maybe RNG (or something else) would be better suited.