Receipt not CC-ed for offline donations
Coming from a different angle, perhaps you could use CiviRules (along with the added extension Email API to enable CiviRules to send emails) so that any time a contribution is added, you could send an email to your bookkeeping address.  (And thus you wouldn't need the CC or BCC for the contribution pages).

Out of the box, you can set conditions for different financial types.  Another option: if you didn't want to go to use CiviRules for every contribution, just the off line ones, you could create a custom field for contributions indicating off line, and then use a CiviRules condition of Field Comparison for your new custom field, so emails get sent via CiviRules only when the back office is filling it out.