Well, a clam would be useful before ten turns are over if you only wanted to alchemise water gems into astral pearls (you would need 10 water to get 5 astral via alchemy, and so a clam is better after five turns in this case). I get your idea though, even if this particular rule does not require an host watch. Setting the hoarding items to cost 160 gems each would make any hoarding strategy slightly counter-productive.
On the same topic, it would actually be possible to enforce a limit on the number of hoarding items per player. They would be set to cost the said 160 gems, and the host would have one nation in the game (and only one province, cut from the rest of the world). Set the Wish spell to cost nothing and make it available only to the host's nation, and the host could still forge enough hoarding items and give them to the players once they pay the regular fee. Once a player reaches the level needed to cast Wish, the spell would be set to normal cost/level, and no longer host-only.
On a somewhat related matter: is there a way to prevent Dominion from spreading for a certain amount of turns? I have been trying to do that, but the best I can think of would be to put Stone Idols in all provinces neighbouring the starting provinces, but that would take several turns at the very least.