Enforcing use of xsi:type

In the Election Markup Language, people may use a variety of identifiers
depending on the type of election, country etc. The schema (XSD) therefore
allows use of xsi:type in the instance so that the correct validation can be
used for each identifier. I don't think that there is any way in XML Schema
of forcing the use of the xsi:type attribute in the instance, so I am
currently using Schematron rules to do this. Am I correct, or have I missed
something obvious?

Before people point out that I am allowing the sender of the message to say
what type of identifier they have, the Schematron (which changes for each
scenario) also defines the allowed values for xsi:type. I would keep this,
but it would be good if the schema itself enforced something as a hint to
implementers to look at the spec and see what they need to do.

Regards

Paul Spencer