This already should be almost available to do. So reasons for using XML were the following:

  Its is native to OpenCV, because all of the data saved was internally in OpenCV datastructures it makes sense to use their own importers exporters to store and retrieve. This also means that we didnt have to write all importers. It is all kept in single flat file. Also to my mind using a db like SQLite makes sense when there is a lot of data to store, where as comparatively there only a few config options and parameters.

However, there is option in libface to retrieve raw config map and then store it in the way you want. So this can almost be done, I just don't really see the point given current low data volume. Having said this it really depends as well on future algorithm for face recognition.