Okay using my method here is a tip on how you can get it working
Create a script /etc/postfix/mail-backup.sh
cat >> /backup_dir/mail-backup
Now for the always bcc use an address like email-backup
In your aliases file have a pointer to the script
All the bcc'ed mail will be stored in the spool file /backup_dir/mail-backup.
NOTE that you can get more fancy with this and split the spool files if you have multiple users by reading the mail headers and getting the to address then store the mail in that users spool file.
Anyway this is just a skeleton for you to build on, haven't got the time on my hands to create you the full solution.