You can place your items in your area of Castle Volkihar, you just have to be careful because everything else will respawn, but the containers and bookshelves are safe. The display cases are all taken up by respawning items, so you cannot do much with them, and I gave up before getting to testing out the weapon racks and plaques. The area I wanted was in the back section of the castle, and nothing worked there, I was very disappointed. The Dawnguard Fort had nothing to really interest me, the weapon rack in our room never worked that well, no display areas and one mannequin I did not really trust, but gave it something anyway just to test out what would go wrong. None of the containers seemed safe. My character Aussie has not even bothered returning it was so interesting.
I never got my hopes up before Dawnguard was released, all I heard was we get a room in the faction area, which really never interested me that much, it was not till I went through the back section of Castle Volkihar, saw the nice big weapon rack and got excited thinking it might be usable later. It was not, and you always have to get to it going the long way around, the area automatically resets and is unreachable from the balcony. My characters have plenty of houses anyway, but something bigger would have been nice.

If the victor had got the spoils Aussie who fought on the Dawnguard side would have moved into the empty castle and claimed it, but since she is not allowed to use anything without stealing it, that was pretty pointless. I will probably test out if everything still respawns, maybe steal it all and get rid of it and still claim it.