No bites at all.
However the plot has thickened. Having more or less resigned myself to using check constraints to enforce conformity to specific namespaces from a xml schema collection containing more than one namespace, I've made a discovery that I do not recall having seen in the documentation.
Have a look at this version of this post:
A gentleman posted a reply with an example and that got me to testing some constraints when I noticed what I'm going to have to call a bug.
the xml .exist() method dies when you point it at a schema namespace containing multiple root elements. The full description is at the location above. If you try to execute the method on typed xml it errors saying that no such element exists in the namespace, if you execute it on untyped xml that includes an xmlns dclaration and the namespace is registered in the schema collection the method always returns 0, regardless of the truth.