This is easily possible. The Paparazzi throttle control has a setpoint (horizontal flight at cruise speed) and adjusts throttle for height control. This method leads to nicely constant airspeed, once tuned well. We flew a lot and also very demanding flights without an airspeed sensor with perfect results. We weren't even able to optimize flight performance using an airspeed sensor. For simplicity and reliabilty I therefore even recommend to do without an airspeed sensor, but rather to invest some time in good tuning.