Here's what I've learned. Not earth shattering, but not completely obvious, either.
First of all, highly selective colleges, at least some of the time, are in competition with each other. In recent years Ed interviewed a boy who was accepted by two Ivy League colleges; his parents then negotiated a better financial package on the strength of the second acceptance. Speaking hypothetically, Harvard doesn't want its admits going to Yale, and Yale doesn't want its admits going to Harvard. (I'm not saying those were the colleges involved in this case.)
Second, one department or area inside a highly selective college or university may be faring better than another. That is to say, one department may be losing a disproportionate share of admits to another highly selective college or university than other departments are losing. I gather admissions departments track these things.
If that's the case and your child is interested in that area, he or she might have a better chance of being accepted than the global admit rate implies.
I don't know whether this information is "actionable," but I think it might be. For instance, it might be possible to find out figures on the number of majors a department has had going back over the past 10 years, say. If the department has seen a drop in majors, it is probably looking for students. (Obviously, academic departments don't make admissions decisions. Nevertheless, it appears to me that colleges want all departments to have students.)
I suspect college magazines may have useful information to share as well. Retirements of famous professors, for instance. When a well-known professor retires, the department loses a drawing card.
Another thing to look for: new programs being developed. It strikes me that new programs might imply a higher admit rate early on for interested students. And new programs will almost have to pull majors away from other departments, which may shift the odds for students headed towards those departments as well.
No other thoughts at the moment.