I don't know what you standard is for "made it work," but it has been done for school in America and people are selling equity for other purposes right now. Calling it slavery is a PR problem, not a legal one. Alex Tabarrok gives several examples. In particular, Yale tried it in the 70s and it failed, perhaps because they didn't try to collect the debt. Then he gave a recent social entrepreneurship example, which is probably besides the point. Selling equity is common currently in the music industry. A firm looking to buy equity in athletes recently signed one.
The music and sports examples may avoid the theoretical problem of people declaring bankruptcy immediately out of school, but that wasn't the problem at Yale. The Yale example ran for decades without running into legal problems or people calling it slavery, but the debtors got annoyed. It was canceled in 1979, I guess because it made people unhappy. Part of the problem was just high interest rates in the 70s. The music and sports examples haven't lasted long enough to see if they run into PR or legal troubles.
http://www.theguardian.com/world/2013/nov/12/occupy-wall-street-activists-15m-personal-debt
A collection of Occupy activists recently bought over $14,000,000 in personal debt for $400,000.
Normally, debt-buying companies do this with the intention of collecting the money from the debtors--Occupy did not, and I was struck by the lopsidedness of the figures.
A number I see often in the high-impact philanthropy world is $2300 to save a life (with plenty of caveats). At Occupy's rates, that would buy roughly $80,000 in debt--enough to get two or three families out of a hole that would otherwise render them bankrupt.
By itself, this isn't enough to be better than mosquito nets or deworming. But the thing about personal debt is that, thanks to interest payments and stress, it prevents people with high earning potential (compared to an average African) from making decisions that would optimal were they debt-free--like finishing college or buying a used car so they can take on a higher-paying job.
My idea, though it's a tentative, spur-of-the-moment thing:
Why not found a charity that acts like a combination of Vittana and Giving What We Can, freeing people with good prospects from debt in exchange for their signing a contract to donate a small portion of their future salary to charity?
A few issues that come to mind:
1) Occupy bought a lot of medical debt, which this company wouldn't, and other types of debt might be harder to buy.
2) People who have decent earning potential have more valuable debt, since they're more likely to pay it off later. (On the other hand, freeing them of interest payments might help them get into a better position for repayment.)
3) The idea is a lot like micro-lending, and organizations that offer that service don't have a great track record (though some have been successful).
4) People just freed from debt might not be in a position to donate much salary/might be unreliable. (Deferred payments until college is finished/the new job is had could be helpful here.)
5) There might be (well, almost certainly are) difficult legal issues with finding information on people in debt before you actually own their debt.
Are there any other obstacles you all can think of? Other features of the charity that might make it more effective? How does it sound as an intervention that increases the world's productivity in the long run, stacked up against other such interventions?