Is this possible, and how hard would it be?

[Foodstamp]

Actually you can sorta do both the things you are trying to do.

For the gem generator. Create a unit that is an item that is recruitable in the province with the #unique tag. Imagine something like the fountain for example, that produces 10 pearls.

There is a way using mod commands and map commands that you could also give yourself a unique item in a province.

Using mod commands, create a #unique unit (Say a pedestal or something). With map commands, place this unit in a hidden province with the item you are wanting to associate with another province.

In the province you want the item to spawn in, create a magic site that allows the unique item bearer to be recruitable.

Here is how it plays out:

Whoever gets to the province first can "recruit" the item bearer (Pedestal, weapon rack, whatever) and take the item from that unit. Recruiting a unique unit makes the unit appear in the province, no matter if it is already somewhere else on the map.

So the player who gets there first gets the item. Later on, if the province is conquered by another player or the AI. They can recruit the unit bearer, but the item will be gone.