Is it possible to find missed loot in d'Arnise Keep after completing the quest?

I'm playing through the game and I didn't make any saves before I started the quest to clear out the Keep. When I finished, I found that I had missed the Flail Head [Cold] and thus couldn't complete the Flail of Ages. When I went to the location in the keep where the cold head was supposed to be located, it was mysteriously absent.

Is it possible to collect loot that I missed after I've completed the quest to clear out the keep? Or am I hosed?

P.S. My main is a Fighter class, so I was offered the opportunity to take ownership of the keep -- which I accepted. If that matters.

Once you take over the keep it technically changes into a different zone.

For instance before it is your stronghold it is

AR1300: De'Arnise Keep Outdoor

Then after you make it your stronghold it is

AR1304: De'Arnise Keep Outdoor (Stronghold)

This should explain the problem. In short, yes you are hosed unless you want to replay the area or cheat the item in and pretend it never happened. Sorry!