My conversation with Syd Bauman yesterday touched on linking non-XSD schemas into documents for simpler validation. All the approaches known are "unofficial" or ad-hoc, and seem to be based on processing instructions such as this one, which oXygen uses:

<?oxygen RNGSchema="../new_schemas/imt_1_5.rng" type="xml"?>

Syd suggested links (this and this) to other approaches; I need to look at those, and decide which we will include. It may make sense to include all of them, or pick one as preferred.