Hiring based on IQ tests is illegal, but hiring mostly from certain universities is not.
I can easily imagine that abject poverty would mess with SAT scores (or probably even prevent someone from taking the SAT), but for the majority of the country I would expect the SAT levels the playing field rather than hurting it. For example, I live in a rural town in Missouri. No one here is rich, but the majority of the town is not in poverty. I can't think of a better way than the SAT for a precocious child here to get into a top-tier school.
The value of the alumni and professional network of these lesser schools, as well as the brand name, is lower than the top tier schools, but the parents often did attend a more elite school, and their network offers inter-generational benefits.
It's interesting that state schools like Michigan accept a large number of less competitive in-state students, thereby providing the sort of springboard you mention, while also attracting a top tier of students that is on par with those attending Ivy League schools.
There's no reason that you couldn't be sued the same way if you're only hiring from a certain university and that leads to excluding people from a protected class.
But there's a big difference between something that existing legal precedent has ruled as strongly likely to be illegal for hiring (IQ testing), and a currently widespread hiring practice that may at some future date suffer the same fate (hiring from particular schools). The legal risk of the former is extremely high, while you would have to be extraordinarily unlucky to suffer damages from the latter. Even if the courts do so rule in the future, you'd have to win (lose?) the lottery and be the one or two companies that get taken to court in that test case.
Citation pls?
It would be legal if you can prove in court that IQ is reasonably related to the job.
Edit: Oh yea, don't believe me? Here's a link. Apparently I got it backwards though. :)
http://www.educationplanner.org/parents/act-sat-scores.shtml