As everyone who participates in strat sieges is aware, the defending side frequently ends up with so many different items that they cannot select them at will. The solution I propose is to allow fief owners to sell gear whether they are in the fief or not, HOWEVER; they cannot buy gear and they cannot transfer gear and they cannot transfer money, any money gained from selling gear will be stored in the fief itself and so can be taken by a successful attacker. This way defenders can clean up their item list a bit without removing any possible spoils of war.
I of course have no idea if this can be coded easily or at all, but I figured I would at least offer up a potential solution that in my eyes would be fair to both attackers and defenders.