Some amount of bleed through trade routes would be interesting. Cities near a foreign source of Iron would be able to buy small amounts through private trade, if any is occurring, so having it go through trade routes (perhaps in return for a larger gold bonus from said trade route) would be realistic, and pose interesting challenges/choices (want your neighbour to have no access to your horses? Close borders, ban trade, or deal with it). If this came with increased control over trade routes, then this could also be interesting. . .
Great work mate