As far as implementation, phase 1 is available in 2.6.0. The main developer working on the App API is Jean-Louis Martineau. If you're interested in the API, please subscribe to the amanda-hackers list and start up a conversation there.
Your second paragraph suggests a misunderstanding, though -- in the Amanda sense, an application is a utility that knows how to convert a particular kind of data (filesystem, database, Exchange DB, etc.) into a datastream that Amanda can place on tape, and how to handle that datastream during restore/recovery. So if you were to write an application, Amanda would invoke your application, not the other way 'round.
Storage Software Engineer