XSD: Adding attributes to strongly-typed "simple" elements

Is there some sensible way to have elements with strongly-typed simple-types and also attributes?

Okay, I have an XSD schema which has a million (er, hundred) elements that might look like this:

<xsd:element name="DocumentDescription" type="xsd:string" />
<xsd:element name="DocumentDateTime" type="xsd:dateTime" />
<xsd:element name="DocumentSize" type="xsd:int" />

That's dandy. However, I really want all of these elements to also have some common attributes on them like, let's say, "format" and "isVisible". i.e. have a schema like:

<DocumentDescription isVisible="true">doc description</DocumentDescription>
<DocumentDateTime format="dd/mm/yyyy" isVisible="true">1/1/2008</DocumentDescription>
<DocumentSize format="0.00 KB" isVisible="false">5403</DocumentSize>

I could do it manually, and horribly, by adding all such attributes to the XSD when I generate it, something like this:

<xsd:element name="DocumentDescription" />
  <xsd:complexType>
    <xsd:simpleContent>
      <xsd:extension base="xsd:string">
        <xsd:attribute name="format" type="xsd:string" />
        <xsd:attribute name="isVisible" type="xsd:boolean" />
      </xsd:extension>
    </xsd:simpleContent>
  </xsd:complexType>
<xsd:element name="DocumentDateTime" />
   ... etc

...but in an ideal world I'd rather define it as a complexType:

<xsd:complexType name="customType">
  <xsd:complexContent>
    <xsd:extension base="???">
      <xsd:attribute name="format" type="xsd:string" />
      <xsd:attribute name="isVisible" type="xsd:boolean" />

...which means I could just do:

<xsd:element name="DocumentDescription" type="customType" baseType="xsd:string" />
<xsd:element name="DocumentDateTime" type="