[Gandalf Parker]

The beginning assumption was that the ally felt the chalice was too precious to loan you by messaging. The scout thing sounds cool. I suppose you could be really REALLY careful to check the stealth level of the unit you are using (altho few people I think ever click the stealth icon and check the stealth level). Then you would trust that the other person has remember to not load the PD (most people put at least 1 automatically after they take a province as recommended in the game tips). And Im not certain but this game usually maintains a D&D feel of always having some slight chance of something happening. If nothing else there is always random events which can suddenly add PD on a province.

All in all the castle method would be safer for the person who was paranoid enough to want it to stay in their possession. Harder to do, but safer. Now someone needs to test and see if it can work. Id love to see a one-stop list of all the things that can be shared between allies. Scales, globals, item effects. Im sure that most will be listed as bad effects by the first poster. But in this game you can often get creative and find one nation or situation that can use that to an advantage.