The validation being done as the user is editing a MED is done using the ModelExtensionDefinitionValidator (MEDValidator). This validator tries to recreate in code what the MED XSD does when the MED file is parsed. The MEDValidator should use XSD validation libraries to perform validation to guarantee that MED problem markers agree with those reported in the editor.

Looking into this a bit I found that XSD validation libraries only validate entire documents. So to validate just an element value or simple type value does not seem possible. One possible way around this might be to create XML fragments to match the smaller sections of that document that require validation. Research still needs to be done on the feasibility of this idea.