I've been thinking about this thread a bit. I think there may be separate questions. One is how to determine access to a program or a course, and the other is the content of of the program or the course.
As far as courses are concerned, I lean toward admitting nearly anyone. Examples: I know a college that admits 70+% of its applicants. The attitude there seems to be, You want to be an aerospace engineer or a physicist? Fine. Give it a go, and remember that we won't water anything down. They don't, and half of a given freshman class doesn't make it to the first day of third year. My impression isn't that this college is deliberately weeding people out so much as simply running the courses the way they should be run. It could be argued that many students shouldn't be admitted, but the reality is more complicated: I've been told that a lot of the students fail in 2nd year because of laziness.
Alternatively, I took a course in modern physics a year ago and wasn't qualified on paper. The instructor let me take the class anyway and gave me no special treatment whatsoever. I got an A. On paper, I looked "unqualified," but the reality was different.
So those are two examples where qualified people can't handle it and an unqualified person could.
My suggestion is to worry less about who gets into the gifted program and more about what it will cover. By this, I mean, in-depth exploration of ideas that are difficult to grasp. So for middle-school-aged kids, an idea would be an exploration of the scientific revolution (which would include lessons on the geocentric model of the universe, how Copernicus came up with the heliocentric model, as well as Galileo's and Newton's contributions). This stuff isn't easy, but it's the kind of material that gifties eat for breakfast.
Another idea is to run an language class. They did that with my class when I was in 4th grade, and it was great. Language Arts was too easy for a few of us, so we got to take Spanish lessons 2 or 3 days a week instead.
You'll end up with some bright kids who aren't gifted regardless, in part because of politics and so on, but if the material is aimed at the most intelligent students, so what? The ones who aren't interested might bail out, leaving the program with a group of bright and gifted kids who'll really get something out of it.
