I shouldn't think you need an arbiter. If an evil nation sends a message to another evil nation they are taking quite a risk in assuming the recipient will also be up for breaking the rules. Especially since the community here is generally pretty honest.
You have different people signed up for that game and some of them are not long. But arbiter would be rather to stop any temptations. If no one is checking then risk is not big.
How will nation placement work for the good nations? Specifically, I'm wondering about what we need to think about as far as overlapping dominion effects. And of course, it will be a real tragedy if one of us was domkilled by our comrades.