Investing in Harvard Graduates (for real)

Richard Vedder offers some fairly radical ideas about how higher education financial aid could be different, particularly at elite schools:

Rich schools like Harvard, Yale and Princeton should let students in for free in exchange for a share of student earnings beyond subsistence for X number of years after graduation. In other words, Harvard should buy equity in the "human capital" that it allegedly creates, include it as an asset in its endowment, and there should be no student loans. Alternatively, students should be given the option of paying tuition now with no future obligation. However, I see no reason why Harvard, Yale and Princeton charge any tuition at all given that they earn at the minimum $75,000 per student in sustainable endowment income annually. There is a fairly decent case that can be made that, given the huge value of tax exempt status to them, these should not be allowed to charge tuition, although I would not go that far.

I probably would go that far. Barring some kind of catastrophic collapse of the nation's financial markets, it seems likely that at some point in the next 10 - 20 years, at least one university--probably Harvard--will reach the theoretical limit of the size of a non-profit university's endowment beyond which some combination of internal and external pressure makes charging tuition untenable.

On the other point, I wonder how much a sufficiently-sized group of Ivy League freshmen could raise per person on the open market if they securitized a portion of their future collective income over a given time period -- a percentage big enough to be worth buying but small enough to avoid massive disincentives to earn? Is that even possible, legally?