I'm not a developer for MPAtool but I've been playing around with building a music related app. In WP8 you were able to access the MusicLibrary using the WP SDK and get the song information. Unfortunately, since Xbox Music is a stand alone app you don't have access to its contents through the SDK, and in fact the old way of getting the song information has been slowly changing since GDR1. Before GDR1 I was able to get a users entire collection (Downloaded and Streaming) but that changed after GDR1 (or 2 can't remember) so that I can only see songs on the device. In order for it to work for your whole collection my understanding is they would have to redesign the app using the new Xbox Music API pilot which allows authentication and access to a users music collection data (song, album, playlists, etc.) stored in the Xbox Music service.
So short answer is the current version of MPAtool might work for downloaded music in WP 8.1 but if properly updated I would think they'd be able to make it work for both local and streaming.