How to write XQuery for XML exchange to examinate message header?

Issue

Sometimes it makes sense to use XML exchange for non-XML messages and have bindings checking message headers rather. Use cases are for example:

• using also other than equal comparison in message headers
• requirement to send some messages to one address string / exchange such that the exchange has XQuery requiring bindings but also binding with much simplier binding filter to route non-XML messages

In such cases, how to write XQuery against message headers? Documentation shows just XQuery against message XML-formatted body.