Make School (also YC, W12), has offered income based repayment since 2014 for our college program www.makeschool.com/product-college. There are now 3 cohorts of students who have gotten jobs and paid us under this model and a 4th is entering this fall.
Ask since I've looked at this model before and while I'm not able to find it, I recall court cases that ruled against this type of finical agreement.