The current project is implementing a validator only, with no access to a schema compiler.

The xsi:schemaLocation and xsi:noNamespaceSchemaLocation attributes allow an instance document to reference the location of a (source) schema document containing a schema for the instance document. Because this validator has no access to a schema compiler, it cannot implement this capability.

This isn't a fundamental show-stopper. Support for these attributes isn't required for XSD conformance, and many applications avoid using them (there are a number of arguments against using them, not least that when you validate a document, it's because you don't trust it to be valid, and if you don't trust it to be valid, why should you trust it to tell you where the schema is?)

If we had a schema compiler written in XSLT, then it would of course be possible to interface this.