Well, as I'm going with Despotify library, unless someone extends libdespotify it's probably not going to happen. libdespotify by itself decodes ogg streams and hands forward pcm data, so it should be encoded again to be reasonable to store somewhere. Extending libdespotify to download just the ogg won't probably be accepted to upstream (my guess, I haven't asked) since then it would be even more easy to download stuff from Spotify. (not that it's too hard now either)

But aside that, it wouldn't be too complicated to cache the data to device and use local version of songs.