> This would likely result in STEM and highly-qualified students lobbying to have interest rates take into account declared major or GPA or some such thing.

I think you're assuming that lenders care what kind of degree you have. Someone who takes a few gap years to establish savings and credit will end up with better interests rates. As it should be. Kids should rack up a few years in the real world before they drop 80-200k and ruin their lives with debt.