Right now I'm just daydreaming, because this project would require about four €50-€100 sensors, but I'm pretty sure it can be done. In fact, I found 2 existing project already.
The dIMU seems the best gyro out there, but from what I gather from the 2 projects I found, it's rather hard to get accurate and drift-free readings. However, my slowstick is pretty much self-stabilizing, so I don't think it needs a rock-solid gyro.
Contrary to what you'd think, the elevator is mostly concerned with velocity, while the motor is mainly concerned with altitude. Thinking about it like this frees us from having to keep the plane level, all we need to do is maintain speed and altitude.
The dGPS module provides these two, plus a means of navigation, although I'm not sure about the accuracy and refresh rate. Alternatives would be a Barometric Sensor and a pressure sensor set up as a Pitot tube, still no clue about accuracy.
A manual override is absolutely required. While it might be possible to do at a hardware level or by messing with the NXTServo, in terms of weight, communicating with the NXT would be ideal. Bluetooth is not up to the job, but maybe NXTBee PRO is?
I read the NXT Rechargeable Battery is lighter than AA batteries, is that true? It's surely bigger.
Might I go completely crazy, I could try to use the ultrasonic sensor to coordinate the last 2-3m to and from the ground. I haven't seen anything that has a longer range.
One last thought: Maybe I can get sponsorship for the sensors? Or just make some money...
http://studl.es Mindstorms Building Instructions