Logistically speaking, before we jump the gun and start shooting for Miss World or Miss Universe, lets look at the way these pageants are organized. For a country to send their representative, there must be a national competition, whose results are accepted by Miss World/Miss Universe organizers.
So the first step is to set up a Miss Pakistan Organization. Anyone can set it up, provided there is enough interest. "Miss America Organization" and beauty contest, for example, was setup in 1921 by Atlantic City businessmen just to increase the tourist season. The government doesn't need to do or not do anything in this matter. So at this point it is hard to say, in Pakistan, whether there is a law prohibiting such a competition. We do have all sorts of fashion shows, and super models parade around all over the place. But never any serious effort was made to galvanize those shows into a bona fide Miss Pakistan beauty contest.
From here we move on to the other issue, which is, whether Pakistani population will accept a Pakistani girl parading around in skimpy clothes in a Miss World competition. We don't even allow our girls to swim much in international competitions. There is some degree of hypocrisy involved here. A lot of people in Pakistan, who have the opportunity (dish/cable) will be glued to their TV sets to watch such competitions and enjoy and debate endlessly as to who should have won... but at the same time, will express disgust if someone even mentions Pakistani women participating there.
Also, people in Pakistan indulge in so many un-Islamic activities anyway, so religion is probably not a good excuse, but to do it in a high profile event, when theoretically the country was conceived as an Islamic state, still provides enormous reservations in the minds of most people. For some, it will probably defeat the whole Two-Nation theory.
Also to note, that even in India, which is a secular country, there is bitter opposition to these beauty pageants for being alien to the cultural values of Indians.
While it is true that such decisions may be left to the individuals, but the title of Miss Pakistan, would arguably represent the nation of Pakistan, so public opinion, including that expressed in this forum, need to be heeded, to a certain extent.