script to download labels from gmail and apply them on offline copy of your messages (as additional header), and companion script to copy them to from one GMail account to another
The "restorer" script is really a copier -- it copies labels from one account to another, instead of restoring labels from the backup. It should be renamed, and a real label restorer should be written.
When downloading labels in the sync script, it should at least have an option to warn about any messages in the maildir that are not in the gmail account. For some cases it might be good to have an option to move them to a different (trash-esque) maildir.