Ugh, encontrei minha própria resposta na página 460.
Sim, inclua o modificador de destreza se estiver sendo mantido por uma criatura. Da página, a ênfase é minha:
General descriptions include notes on activation, random generation, and other material. The AC, hardness, hit points, and break DC are given for typical examples of some magic items. The AC assumes that the item is unattended and includes a –5 penalty for the item’s effective Dexterity of 0. If a creature holds the item, use the creature’s Dexterity modifier in place of the –5 penalty.