The official documentation for the app-schema extension has been revamped! There is a lot more to look through here - absolutely a must-read for anyone interested in using GeoServer to serve GeoSciML features.
I've been in the midst of a daily back-and-forth email discussion with the app-schema developers on the GeoServer-Users mailing list. Reading through the conversation may be helpful, as they've already resolved a number of issues I've had. Here's a link to the thread:
I'm almost done building a complete mapping file that will output GeoSciML MappedFeatures from polygon features in a database that follows the NCGMP09 schema. This schema, developed as part of the USGS National Cooperative Geologic Mapping Program, is a proposed standard database schema for the publication of single digital geologic map. An ESRI Personal Geodatabase implementation of the NCGMP09 schema is available on the website linked above.
At present, there is no user-interface component that helps you configure a complex-feature WFS in GeoServer. I'm sure that it is being worked on, but in the meantime, the configuration is done manually, by building the appropriate directory structure and putting XML configuration files in the right places.
This is an example of the directory structure that has to be in place.
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.
There have already been successful implementations of GeoSciML Web Feature services, using a few different approaches.