Shahin jaan, I'm not failing to recognize the difficulty of that process at all - it is going to be a very tedious and nearly impossible task for sure.
However, my argument is that it's MUCH easier to ban Islamist ideologies during the drafting of the constitution which may take a few months, than it is to ban Islamic parties not only from the drafting of the constitution, but also from the entire political process for decades and centuries to come.
I am repeating myself now but it is next to impossible to ban Islamic ideaologies in a perdominately muslim country in which Akhoond and Masjed has a substantial influence and power from the constitution.
I would personally have no problem with having Islamic representation in the committee or group that would draft the constitution, much like I would not have a problem with having any minority ethnic, religious and special interest groups represented. At the end of the end of the day the constitution has to be representative, but all these groups have to recognize that they will not have rights or special privileges that go above and beyond any other groups - that's the non-negotiable condition to participate. Once that condition is understood, it becomes impossible for any one group to exert its will over other groups and the constitution will automatically become a secular one.