I like this idea

Ezra Klein: Educational Incentives (Don't Go To Law School!)

Simple solution -- have the monthly student loan payment capped at a fixed fraction of your monthly gross income, and adjust the term of the loan accordingly.

They actually do this somewhat, but to a limited extent. They allow you to pay off at a lower rate fixed to your income, but only for a period of time; they expect that at some point you will make real money. Doesn't always work like that.