I like that idea, Rhialto

.
On the matter of the UN, I think it should count as
just the Security Council, where the builder and any other civ of a certain level of Economic/military power are Permanent members (perhaps Permanent Member status can be limited to 5 or 6?) Permanent members can:
(1) Ban a certain unit type-as suggested by Rhialto.
(2) Declare Economic Sanctions.
(3) Ban a resource from use.
(4) Send Peacekeeping forces to the cities of formerly warring civs.
(5) Bring down a resolution authorising a joint declaration of war (with all UN Permanent members required to declare war on the offender).
(6) Admit a nation as a member of the Non-Permanent Security Council.
(7) Dismiss a Permanent/Non-Permanent Security council member.
(8) Upgrade a civ from Non-Permanent to Permanent status.
(9) Appoint a civ leader as 'Secretary General'.
The only way for a permanent member to lose his seat is by a unanimous vote of the remaining permanent member civs (or an absolute majority of the entire security council), or by falling below the minimum 'Power Requirements' for membership. The only civ which cannot lose their status is the 'Owner' of the UN. Also, the owner of the UN-and the Secretary General- have Veto power on any votes brought down by other Security Council members.
Anyway, thats my feeling on this matter except that, like DH_Epic and others, I believe the UN and the Diplomatic victory should be completely seperate-except that Building the UN and/or being Secretary General should count towards a diplomatic victory (a victory condition which ought to be points based.)
Yours,
Aussie_Lurker.