what about giving factions from the start a unique building that would represent a royal/republican treasury? it could be located in each capital, unless there are facts that some had it located in other sites. capturing the city and destroying the treasury can represent looting and should make a lot of money, the way demolishing of some temples and other "uniques" work now.
iirc some factions had their treasury in the temples but not every peoples had it this way.
not sure if a building can be transferred when the capital moved? that is not mandatory though, since afaik founding/moving a capital wasnt necessarily a reason for moving the treasury there.