This paper proposes a new technique that enables open innovation in mobile platforms. Our technique allows third-party developers to modify, instrument, or extend platform API calls and deploy their modifications seamlessly. The uniqueness of our technique is that it enables modifications completely at the app layer without requiring any platform-level changes. This allows practical openness---third parties can easily distribute their modifications for a platform without the need to update the entire platform. To demonstrate the benefits of our technique, we have developed a prototype on Android called Reptor and used it to instrument real-world apps with novel functionality. Our evaluation in realistic scenarios shows that Reptor has little overhead in performance and energy, and only modest overhead in memory usage that ranges from 0.6% to 10% for the observed worst cases.