That could just be a slight modification to the item/scroll system. Replace identity of the sender object with bonds, set an automatic equivalent price in gold.
But something about this that might need to become addressed is the possibility that bankers will turn by and large into action bankers, and be in the frontlines exchanging gold for bonds regularly during wartime. My impression of a banker is that of fat greedy coward, and while that archetype could exist if this feature were implemented, I get the feeling that a realm engaged in a long-distance campaign would probably protest out any such banker in favor of Action Banker!.
After all, that guy has a lot of responsibility to keep forces abroad for longer, and one of the most limiting factors that prevents it is not the morale drop (can be fixed with mercenary and entertainment in allied regions) or equipment damage (also can be repaired by allied smiths), but gold, which can't be exchanged for bonds in allied cities. So finally we get the answer to our troubles, go Action Banker!! He'll be kind of like the less potentially popular action trader who breaks sieges.
