Less a future problem with the UN, and more one with the courts. Some one, some nation sits out the initial explorationa and utlization of Mars. But when others establish themselves, or makes a big pile of money with a discovery, the one who remained to the side begin to cry out for their "share" of others work and efforts. They could point to the illconsidered treaty in a court challenge.
Maybe that's what triggered the Fundamental Declarations of the Martian Colonies in the first place.
How quickly would the governments of Earth get off their asses about colonizing Mars, if it's discovered that a meteor was going hit Earth and end all life on the planet in the next couple of years?
They would surely forget all about Mars and concentrate on neutralizing the meteor threat, a monumentally simpler task than trying to establish a viable, self-sustaining colony on Mars.