I think that would require national legislation. make paying players under the table an actual crime - not just an NCAA rules violation. Of course, this would have to survive court challenges, but if it was done in conjunction with the NCAA adopting the new "athletes are employees" structure, I think it would be possible.
if under-the-table payments were illegal, then there could also be lots of tax implications - filing false business records, etc. make the penalties strong enough, and that could serve as a deterrent. besides, if Pennybags could make a direct donation to the school, and the school could use those funds to compensate athletes, that would - I think - lessen the need to go under the table.
and kudos for "Rich Uncle Pennybags." I wonder how many people get that reference.