Thanks for all of the replies.

Andrew:
It's not really "longer than a year" though. Under usual circumstances, an auditor takes 3 to 6 months to return their year-end adjustments. Add in some unusual circumstances and the timing is a lot tighter.

Support:
In versions prior to 9.0, users made a copy of their Ledger data and reserved it, thus allowing Prior Year entries whenever required. If the client has the same timing issue each year, your suggestion of having several copies of the data would work, though it defeats one of 9.0's greatest features.

Ryan:
I'll talk with the firm involved and see how important this is to them. Thank you for the suggestion.