Andreas Bolka pointed out that the params element is not necessary for method calls with no parameters (depending on how you interpret the XML-RPC spec).
I also modifed the valueType definition to partially handle the case when the type sub-element is not used. The mixed="true" attribute and the minOccurs="0" for the xsd:choice allows:
but also allows:
<value>erroneous text<string>some text</string></value>
Unfortunately I'm pretty sure this is as close as you can get with XML Schema.