Nearly all of those concerns are fairly easy to handle from an accounting/bookkeeping perspective if GOG follows anything remotely close to GAAP accounting principles. I understand that the specifics of the accounting regulations that they are subject to may differ, but I'd be surprised if they did so in a substantial way. Upon receiving the payment for the bonus code, GOG would book that as revenue with an attendant payables journal entry. They would then pay the appropriate publisher upon redemption of the code (assuming that there was no expiration date on the code) and reconcile the journal entry. If there is an expiration date and the code goes unredeemed, they keep the revenue and again reconcile the payable journal entry.

Again, assuming that they are accounting near-GAAP the year-to-year carryover is a matter of making an accrual based on what they feel is the likelihood or purchase/redemption occuring in differing accounting periods.

Now, all that said, there may be other business considerations that prevent the offering of generic codes. But I can't see why accounting would be an insurmountable obstacle.