Within "Preferences | Languages & Frameworks | Schemas and DTDs | Default XML Schemas", provide a means to associate any file type that is associated with XML in "Editor | File Types" with a default (implicit) XML schema(s).
A use-case for this would be the Android Layout files *.axml files from Xamarin Android framework (Note: Not XAML but AXML). The android xml layouts are assigned an XSD that is packaged with Xamarin Studio and can be seen in Xamarin Studio's preferences in "Preferences | Text Editor | XML Schemas". Without the XSD file, the developer is unable to use many IDE features.
By providing this feature, it would allow better support for Xamarin development while not developing a solution specific to Xamarin.