Meta-validation Re: [xml-dev] dynamically generated XML Schema?

Curiously, has anyone attempted to do meta-validation to
realize conditional validation ? (much along the same line of 
thought as template metaprogramming in C++ to realize 
conditional compilation) 

What I mean is that can conditional validation be facilitated 
through mechanisms such as "Substitution Groups" 
and/or "xsi:type" ?

If, workflow/application states infoset are encapsulated as 
elements with different specific types at different point
of the workflow/process rather than its state information being
encapsulated in the same type regardless of its processing
state, then I think, we can easily achieve conditional validation 
with the current xsd semantics. 

Any thoughts on this meta-validation approach versus say
dynamic schemas ?

Kuan Hui