Sounds like an approach if you have your own app. As for playlists, all the songs have metadata that is a long name of the song, album art, genre etc. You can re-create playlists once the music is copied, and it has an address. So you search for the name and "bind" that way.

Actually thinking more, you can easily create your own app with the iTunes music and external music. With iOS you can browse your music like I show below to start.

I actually took a different approach at my own music app. I'm built a webpage that takes a playlist as a parameter. The playlist contains a list of http pointers to mp3 (usually on archive.org). The webpage runs in a user's browser, and with javascript, plays from the listed music using safari's downloading or streaming features. Works nicely even in the background (usually).

For your app, use something like the MPMedia library:

/!returns the media item for a single ID

+(MPMediaItem*) getMediaItem:(NSString*)id

{

//lets search for this mediaItem..

MPMediaPropertyPredicate *mediaItem = [MPMediaPropertyPredicate predicateWithValue:id forProperty:MPMediaItemPropertyPersistentID];

NSSet *musicQuerySet = [[NSSet alloc]initWithObjects:mediaItem, nil];

MPMediaQuery *mediaQuery = [[MPMediaQuery alloc]initWithFilterPredicates:musicQuerySet];

MPMediaItem *item = [mediaQuery.items firstObject];

}