Unless your opponent activated CA in the battle phase (with Meal in Emmaus or something of the sort) then it would be too late for PP to negate the ability. He can deactivate it, but since the ability was activated in the prep phase, negating it in battle does nothing.
Hidden Treasures was used in the battle phase, so yes, you could get your enh back by blocking with PPriests.
Basically, any Artifact that has had, is having, or will have an effect on the Battle Phase can be stopped, assuming it is negatable, by PPriests. Any artifact that had their effect during the prep phase cannot be.
