Yes in our sample project, look at LanguageDesignerForm. It does this and creates span indicators after validating against our XSD.
To test it, open the Language Designer from the launcher window. The open any sample language definition in the top box. Then add a valid random XML tag at the top but not a tag that is valid per the XSD.
Here's a sample where I add a Test tag:
<SyntaxLanguage Key="C#" LanguageDefinitionVersion="3.0" Secure="True" WordContainsAdditionalCharacters="@_" xmlns="http://ActiproSoftware/SyntaxEditor/3.0/LanguageDefinition">
<!-- String Properties -->
When you hit the validate button, you'll see a span indicator wavy-underline the error.