And some programs actually list where students have come from in recent years.
Dartmouth had such data, but it was for transfer applicants at the undergraduate level. I think I've seen similar stuff for grad programs.
One thing I think you should consider doing is properly recording the data you find. For e.g, math PhD program at Columbia: No. of students in first year, second year, and so on. And among those, how many have their CV listed? How detailed is it? Is there any mention for research projects done at the undergraduate level?
I'm sure you could be more meticulous about it, but if I were to do this again, I'd start with at least that. When I did this way back, it was just to get a sense of where people in top programs (lol, not just the ivy league) came from, and I saw people ranging from schools like the MITP (Moscow) to the IITs (India) to State Us and not very well known schools in the US.
One thing I've learned in my time here and pgre.com is that what *you* do with the resources available to you is far more important than where you do it. Sure, going to MIT means you have even more resources, but in how many labs can you be at any given time? You'd definitely get more flexibility than in a liberal arts college, but if you're at a State U with a reasonably sized department (in your case, math), you'll probably be able to do just fine.
Note that I am only an incoming college student myself, so take this with a grain of salt.