I see that there is an effort at ISO SC34 WG1 to try to get an agreed on way to associate documents with schemas. XSD has its hints like
@xsi:schemaLocation, but it has the flaw of needing the schema to be altered in order to allow the document with the hint to be valid. A PI is clearly a much less intrusive approach, with the model being
Topologi has long used a PI at the top to give the document a label, which can then be used to select a configuration file. It was one of my projects to push ahead with getting such a PI (George Bina from Oxygen was onside too) before my surgical adventures of a couple of years intervened: I am glad that others in WG1 are taking this up.
XMLBlueprint XML Editor version 7 has been released. Among the new features is support for ISO Schematron - Validation and XML Completion, including a Schematron editor. Documents can be associated with Schematron schemas using a processing instruction
I see also that the most recent version of Oxygen XML also supports a good range of the ISO DSDL spectrum: ISO Schematron (including abstract patterns and XSLT2 constraints), RELAX NG, NVDL. Documents can be associated with Schematron schemas using a processing instruction
<?oxygen SCHSchema="file:/C:/work/book.sch" type="xml"?>
Oxygen also have a good short video introduction to Schematron for programmers that I recommend.
XML Mind also supports ISO Schematron, They have a PI for associating RELAX NG schemas, but I didn't see one for Schematron.
However, I was really pleased to see how they supported phases (a mechanism in Schematron that groups patterns so that you run some and not others): according to their configuration guide you can actually use an expression in their configuration file to decide which phase to use: excellent!
The Eclipse Schematron editor plug-in has a PI