This needs some examples that currently exist or have been tried. The link about English student loans is actually about a very similar system, but you don't say that.

Yale tried something like equity financing of student loans in the 70s and was not happy with the results. Selling equity is common right now in the music industry. The elite schools in France (eg, ENS) come with a commitment to work for the government for 10 years (eg, as a professor), but which is often bought out by other employers.