Taxation is not possible without the threat of force. The mighty U.N. Navy, I take it, will be the enforcement arm?
If they are fool enough to pass it, we can laugh mockingly at them.
The IRS has never had a problem enforcing payment of taxes and if the U.S. signs a treaty agreeing to pay taxes to the UN taxes will be payed to the UN. The agencies of collection and enforcement are already in place.