As a regular college student, if someone wanted to pay me money for my autograph I could take it, no problem. If somebody wanted signed copies of the university-published research reports I did as a grad student, that would be no problem.
I understand how paying players formally or via boosters open up huge worm cans for the NCAA, as well as the whole tangled mess that agents bring in, but what exactly would be the problem with allowing student athletes to profit from their likeness the same way as any other student could?
If agents are the concern, simply add some groundrules: you can profit from your likeness with X exceptions...