Hmmm...I think when the Yoshimo thing happened to me, any critical items that he was carrying (i.e. Pommel stone for Equalizer, Gesen Bow stave, Silver blade, etc.) was transferred to someone in my party. Did you check everyone's inventory? If everyone's inventory was full, and the game tried to transfer the items to someone, you may have dropped them onto the ground and not noticed???
If he didn't have the item on him when you killed him, then you're out of luck, I'm afraid.
