It works! Once you get the app-schema extension installed and the tutorial data in place, it works just fine. Make sure that in your GetFeature requests you specify the outputFormat as GML3, as it won't work at all in GML2 (which GeoServer defaults to if no outputFormat is specified).
Also, be aware that the app-schema extension is only supported at WFS version 1.1.0.
Now I can move on to beginning to map my data into GeoSciML features. I'll write some more walkthrough as I put it together.