I've begun work and am seeking review on:
Currently I have simply extended the existing interfaces, and followed the same design patterns that were already being used, but ALR and I had discussed migrating to a Mutable/Immutable type relationship for descriptors.
WebAppDescriptor would be read-only.
MutableWebAppDescriptor would include the builder-methods to update, add, and remove data.
Going to send a pull-request soon.