Sounds great
Any idea where to start looking for how to get it done?
And come to think of it, 20 is too much, even for a large game. 5 makes more sense if the idea is to keep it for flavor only. It will also allow nations not to go out of their way in order to get those hammers. The only difference from 1 is that it's less drastic so instead of turn off the lights they are just dimmed.