Here is an idea for you... you can create hidden resources that serve absolutely no purpose. Make one where everyone has 100 of such a resource. You can increase this resource (I suspect) with the tag above. Set a game event (you get a hero type thing that checks for resource meeting a threshold, once reaching that threshold you can give each player the set amount of resource that you wanted to in a game event (similar to fame increase).

To summarize...

create undetectable resource that everyone starts with a fixed amount...

Increase undetectable resource with tag above (untested, hope it would work)

Define a quest game event similar to CoreGameEvents waiting for a resource threshold to be met.

That game even should give as a reward the fixed integer amount of resource. (be sure to not let them deny the choice)

--- untested, but I think it should work ---

Well I suppose you will have to give some to each player... I suppose you can attach the giving of this resource from the settling of the first city, I've not played with adding resources too much in game.

I suppose you can add this to the champion history abilities as each sovereign must have a history.