Maintenance work can easily be negotiated at this schedule. For example, you can launch a project, and then stay on "retainer" for 5 hours a week for bug fixes. If you find a few projects like this it could meet your criteria.

With new feature development it would be harder. But you could negotiate a weekly fee based on a 4 day work week, and then get so good and efficient that you do the work in 3 days.