I think I may have a better idea. First, I haven't read through the whole thread so excuse me if this idea was already raised or if I get some assumptions wrong.
Instead of a united nations, what we should do is a World Trade Organization. It seems to me that the point of having a group that all (or most) countries join is to make the game feel smaller and more active by increasing interaction between players. The problem with a UN in particracy is that lots of players/governments are going to balk about joining because the discussions seemed to be going in the direction of giving some countries real-life security council-style powers AND giving this thing moderation sanction which would make players feel like they're being coerced into relinquishing their countries' sovereignty (partly).
A way to achieve the goal but avoid the sovereignty challenge is to make the organization about an essential RP element that's currently lacking in the game: cross-border trade. Most real-world international conflicts are over resources, goods, and services -- who makes them, who receives them at what price, and who controls them. Particracy conflicts tend to lack suspense (and are often heavily-scripted ahead of time) because there are few or no life-or-death choices for players to make when deciding whether to engage in a conflict, and where such choices exist they tend to be contrived because players can simply choose to ignore consequences they don't like or wipe the slate clean by pretending previously-acknowledged negatives never happened (I know this has recently been discouraged or outlawed by moderation, but it's still part of the game culture and you probably get my larger point).
So the way to make the game world feel smaller is to force players to communicate across borders. Here's how we do it:
1. Terran Global Trade Organization is established; only governments can join
2. Each member of TGTO can freely enter treaties with other member nations about the level of import/export between the two countries
2.1 These treaties should be simple and include only a lump sum dollar amount of how much country A exports in total goods and services to country B and vice versa; treaties can also specify certain strategically-important export goods (uranium or petroleum for instance) but this shouldn't be done too much because the process and organization need to remain simple and easy to understand.
2.2 The treaties are binding for in-character RP purposes, meaning that the amount of total export-import between of any two countries is accepted as factual within the game world and can't be disputed except through cancellation of the trade agreement; this would be enforced by moderation
2.3 Each member country would maintain an Export-Import Thread which would list their game system-generated GDP, the sum total of all their export-import agreements, and a grand total (game GDP + export - import) which would equal their RP GDP; this figure would be enforced by moderation
2.4 When member countries amend or cancel trade treaties with other countries, they must adjust their RP GDP accordingly
3. Any country that does not join TGTO would have any economy-related RPs ignored by member countries; this encourages countries to join
4. Trade-related disputes would be discussed, debated, negotiated, and mediated in and by TGTO: this is the talking to players in other countries part
4.1 The TGTO would be able to (by majority vote) encourage member countries to adjust certain trade practices and policies but TGTO would have no authority to force any such changes: it would be up to member countries to decide whether to implement TGTO advisory or not and if refused, countries could decide to escalate the dispute outside TGTO
This alternative would be good because it would accomplish the (assumed) goal of creating a central place where players in different countries communicate with each other while having the added benefits of:
1. Forcing players to communicate across borders (instead of just creating a space for them to do it) because if they don't, their economies will suffer
2. Enriching the game and increasing activity by creating logical impetuses and motives for international conflict: if goods don't cross borders, armies will.
What do you think? Say you love it.