dynamically extensible schema

Hello,

I have a question I am banging my head against. I am looking for a way 
to validate instances like the following:

<?xml version="1.0" encoding="UTF-8"?>
<dtw xmlns="core" xmlns:ext="extended">

     <echo message="this is a message"/>

     <ext:echo/>

</dtw>

The special thing is that the potential number of namespaces/schemas 
appearing in instances (beyond the core schema) is undetermined, and I 
therefore do not want to include a reference to the "extended" schema(s) 
inside the "core" schema. Instead, I would like to have a definition in 
the core schema that says something like:

"after the echo element, let there follow any number of elements from 
any namespace whose type extends the type XY" (where XY would also be 
defined in the core schema).

The only meachnism that I found in XML schema that comes close is the 
xsd:any element, but that does not alow me to specify the derivation 
constraint I mentioned above.

Does anyone have a hint?

thanks,
Christian