Tim.. I just used 2 of them that way. I had our Magic cruise booked (for a long time) and went on a Conquest cruise and bought 2 new FCCs on the Conquest. I submitted them (final payment is due next week I think) and they accepted them without a problem. I think as long as you are before final payment you can submit them.
That is a bummer you can't buy them for someone else. They would make great gifts.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007