Well, this is why it seems like a silly idea to me.
If a society restricted to higher-IQ people would be richer, then everybody would want to join it. You'd have to get pretty draconian about keeping people out. Never mind that starting new countries is nearly a fantasy idea in the modern world. (Yeah, I know, if you will it it is no dream).
On the other hand, if we developed ways to biologically enhance human IQ, even a little, I'd see it as a public health measure. There's also eugenics, I suppose, but I doubt most residents in democracies want to be part of a Bene Gesserit breeding program.
And if you wanted an organization restricted to the rational, there's always the possibility of founding a university that admitted people on non-standard principles. Instead of looking for leadership, extracurriculars, etc., which basically selects for affluence and cooperativeness, you could actually just give a battery of tests and require students to write an essay that proves they can think rationally.
Most countries keep immigrants out fairly well, that's not the problem.
The problem is, as you say, that you just can't found a country.
As for a "rational" university, it is probably not big enough to internalize the benefits of rationality.
Really the question is "what is the most feasible group that would internalize the benefits of rationality and thereby prove empirically that rationality works?"
Last Wednesday (2010 Dec 01), BBC Radio 4 broadcast a studio discussion on the question: "should we actively try to extend life itself?" The programme can be listened to from the BBC here for one week from broadcast, and is also being repeated tomorrow (Saturday Dec 04) at 22:15 BST. (ETA: not BST, GMT.)
All of the dreadful arguments for why death is good came out. For uninteresting reasons I missed a few minutes here and there, but in what I heard, not one of the speakers on any side of the question said anything like, "This is a no-brainer! Death is evil. Disease is evil. The less of both we have, the better. There is nothing good about death, at all, and all the arguments to the contrary are moral imbecility."
Instead, I heard people saying that work on life extension is disrespectful to the old, that to prolong life would be like prolonging an opera, which has a certain natural size and shape, that the old are wise, so if we make them physically young then old people won't be old, so they won't be wise. Whatever cockeyed argument you can construct by scattering into a Deeply Wise template the words "old", "young", "wise", "decrepit", "healthy", "natural", "unnatural", "boredom", "inevitable", "denial", I heard worse.
If I can bear to listen again to the whole thing just to check I didn't miss anything important, I may write something on their discussion board.