To issue a fatwa the cleric, immam, ayatollah or whatever must find in the koran and hadith a precedent from the life and teachings of mohammed.
Are they asking the UN to ban the koran and hadith?
In an honest proceeding I agree that this might put the UN in a difficult position. However, considering the UN involvement I would suggest that they would probably specify that the tribunal bend to 'prevailing religious authority' or some such nonsense. It is indeed an interesting quandry but I have great faith in the U.N.'s ability to appease everyone while accomplishing nothing and perpetuating pain,suffering and murder all over the world, especially when the target is the US or Israel. Do I seem cynical? :-)