Why not use a library backup as a library backup?
I have done that for as couple of times when I have wanted to merge my existing CD/DVD data with another library:
1. Restore a backup file (from the library that contains the CD/DVD data)
2. Remove the files that are not a part of the CD/DVD section.
3. Import the other files from the other library by MPL.
4. Import the playlists from the other library (exported manually)
5. Recreate the smartlists (copy the rules from the other library)
It kind of works, but only if the optical discs were previously inserted and tagged or burned on a single PC/MC library instance. We have several PCs and from time to time I would like to merge the CD/DVD data from two or more PC/MC instances. I can't think of any workaround for that.
Some of the data for Audio CDs can be found from the online database (but only the basic fields, no extended tags, cover art links or statistics) and, most importantly, that does not make possible to browse the disc archive before the discs are inserted. Naturally the DVD Video discs and our various burned data discs (home video, recorded TV series, images, old audio files, etc) don't even get the basic library data automatically. (The Wikipedia lookup might help a bit with some popular DVD Videos, but the discs must be inserted and the lookup must done for each individual disc manually.)