Excellent idea! I have used the version for Android and this works quite well. As I have now converted to iOS, I am very interested in the version that you are building for iPad/iPhone. Did you have any success? Happy New Year!
There are a few apps already available which turn an iOS device into a workable joystick/throttle for FlightGear. This is a free (but not Open Source) one I have used successfully with FG: http://www.mungoscott.com/downloads.html