This would be required for implementing non-tearing animation using page flipping based on a double height virtual buffer. It appears that the mailbox interface supports this already ('Set virtual offset' https://github.com/raspberrypi/firmware/wiki/Mailbox-property-interface#set-virtual-offset ) and only the fb driver wrapper would be needed.

Discussion on the issue (the original title is something slightly different) http://www.raspberrypi.org/forums/viewtopic.php?f=67&t=19073 (example code in http://www.raspberrypi.org/forums/viewtopic.php?f=67&t=19073#p231052).