XML Schema: restricted <xs:any>?

[Scott Lamb <slamb@slamb.org>]

I'm writing something similar to Cocoon's XSP (called XFP, for the 
moment). I'd like to make an XML Schema for validation purposes. The 
problem I've run into is dealing with non-XFP elements. I'd like to be 
able to embed arbitrary elements that aren't specified in my schema, 
but I'd still like to have some control over their content. Ideally, 
I'd be able to do something like this:

   <xs:element id="other" namespace="##other" mixed="yes">
     <xs:complexType>
       <xs:sequence>
         <xs:element ref="xfp:attribute" minOccurs="0" 
maxOccurs="unbounded"/>
         <xs:choose minOccurs="0" maxOccurs="unbounded">
           <xs:element ref="xfp:element"/>
           <xs:element ref="xfp:logic"/>
           <xs:element ref="xfp:expr"/>
           <xs:element ref="other"/>
         </xs:choose>
       </xs:sequence>
     </xs:complexType>
     <xs:anyAttributes/>
   </xs:element>

In other words, I'd like to be able to say that xfp:attribute always 
has to precede other children of stuff in random namespaces. That kind 
of thing. From the W3C specs, it doesn't appear that either 
<xs:element> or <xs:any> support this kind of thing.

Is there any way to do this?

Alternatively, is there a XML Schema for Cocoon's XSP available? I'm 
sure they've had to solve this same problem.

Thanks,
Scott Lamb