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.