I actually haven't found a production implementation of APP ...
Uhm, yeah, I've been meaning to mention that; there are no production implementations of the APP. As a matter of fact, there are no implementations of the APP at all. Period. That's because the spec isn't finalized, and part of that finalization process will be the assignment of a URI for the "app:" namespace, which implies that every single implementation today has the wrong namespace for Service and Category documents.
Update: The new namespace has been minted: