Some people make a distinction, in political choice, between decisions that involve establishing a framework within which subsequent decisions are going to be made, and decisions that occur within a framework. The latter would be ordinary political decisions and the former would be "constitutional" choices. This distinction is made to stress the thesis that actors have different criteria for making each kind of choice: in ordinary political decisions, narrow self-interest is foremost, such as getting more votes by getting would-be voters more goodies; but in constitutional choice, there is as it were a wider view, because one is never sure when the constitutional rules might be used against them, so reasonably fair rules are chosen. For example, John Rawls gave a more extended argument and said that "social rights" are just because people, if uncertain about what would be their economic position once the social contract is concluded, would prefer to have such policies.
My quibble is that any informed reading of actual "constitutional choices" would conclude that those who are making the decision do so trying to figure out what rules would be most likely to produce the results that they would like after the constitution is adopted. And it seems to me that this should be right because after all, even if we are not completely sure what the results of our planned actions will be, we always try to work out what's next with our limited information. For example in Forrest MacDonald -- although he argued in a book against the view that the vote of people in the conventions that ratified the US constitution was influenced by economic interests -- describes the series of compromises that lead to what would become the US constitution; the composition of the Senate, initially representing each State equally was composed so to appease small states who thought they would be overridden by bigger states with more representatives. Another example is the Mexican primaries: there is no method for selecting candidates for any election so every time an election comes a method has to be agreed upon to select the cadidate (a sort of "constitutional" choice, although it is made over and over); the upshot is that every man who wants to be a candidate promotes the method that he thinks will result in his being selected.
I suppose what I am making is a claim to knowledge: that it is unreasonable that, assuming rational action one can also say that individuals won't try to figure out what rules will have what effects after the veil of ignorance is lifted. Let me tell you that I am one of those people who wouldn't lament the disappearance of "social rights" that would supposedly be preferred under the veil of ignorance. I am tempted to say that the right general interest that should be considered would tend to limit the powers of government over its subjects very much -- but I am also kind of a cynic, and think that people, in the end, choose what is in their narrow self-interest (although I may be underestimating such things as ideology and simply the preservation of the status quo).