Have you checked the Hoagie's web page for lists of gifted schools?
http://www.hoagiesgifted.org/schools.htmSome gifted magnet type schools (part of a public system) might have testing you could do without a residency requirement as long as you could prove residency by a certain date of the school year starting. You'd probably have to go to the place for testing though. Downside is that a qualifying child based on their scores might go into a pool for a lottery.
Private schools that concentrate on G/Ts might be expensive but you could compare to the international schools maybe they look better in comparison. I have no clue, I'm just throwing ideas out based on how I'd think about it if I had such a cool opportunity.
Another idea is to pick a place that has a high population (greater the chance of a higher IQ kid in the pool...) of diverse people with lots of enriching things to do, and sort of hover around the areas where G/T or otherwise unique and interesting kids would frequent (museum classes and things). For example, the Hunter College Elementary School in Manhattan. If your DS can't go there because K is the only entrance, you could sort of fan out to where the non-accepted kids go (their cut-off is pretty high so kids who don't make it are still very bright).
That's just one example...I know NYC and area are pretty expensive.
My DD was remarking yesterday about a picture and something about so much sky. I realized she's always lived someplace where you mostly just have to look up to see the sky above the buildings and the trees. Except when at large bodies of water... I think if we had the opportunity to do something different for a year, I'd take my DD to a very wide-open place for a change.
