Well, Id introduce a new playlist type.
As in playlists with type = 2 are those “smart albums” 😉
Editing the smart playlist wizard to set another type should be a rather easy part.
Now adding some extra logic to the file scanner that actually doubles up the songs, and justs changes the album name to whatever the smart-playlist name is should be doable.
But you probably want to remember which songs that are, otherwise your library will grow one extra smart album every scan, which is a bad thing.
There is an awful lot of fields in the songs table, but I guess some are better than others.
I dont know about the use of songs.grouping, maybe you can use that field to store the id of the smart-album-playlist that it’s created from, that way you could join from the songs and the playlist to really only get new items, or flush old ones out.
0.02€