Firstly, in case it perhaps helps here is the full quotation:
Then Aragorn took leave of Celeborn and Galadriel; and the Lady said to him: ‘Elfstone, through darkness you have come to your hope, and have now all your desire. Use well the days!’
But Celeborn said: ‘Kinsman, farewell! May your doom be other than mine, and your treasure remain with you to the end!’
I personally feel that Celeborn is refering not to something that had happened or was going to happen in M-e, but outside it.
Elves did not know what awaited Men after their deaths, but it is said in the Silmarillion that with the passing of time Elves and even Ainur would eventually come to envy Men for the gift they receive from Eru.
Is this not perhaps already a proof for this. Is Celeborn not perhaps actually saying "I hope that what awaits you beyond death will be better than all the time I still have to spend on Arda and that Arwen will be by your side until the world is remade." ?
Just a thought.
