Am I missing something here? The steering lock/transmission lock are a security feature of the car. With your setup, you'd need to leave the key in the car all the time in order to drive it - and the transmission and steering wouldn't be locked so your car would be less secure, not more.
I'm not sure about how to determine if the car had moved, but if it were me, I'd get a second GPS unit specifically for determining when the car is moving (mapping software seems to have a tenancy to lock GPS receivers so that nothing else can get see the data). Then you just need something like GPSMAN (its hidden in the list of free GPS downloads
http://www.maps-gps-info.com/fgpfw.html ) to write the data to a log file. Finally, parse you'd parse the log file (I could probably figure it out in VBScript), and look for changes in position that are greater than the inaccuracy of your GPS unit.
I could see having problems with this if, for example you were parked in a city, near tall buildings with very limited GPS reception. Whenever you missed a GPS signal, it would think the car was moving, and shut it off.
Why not just make a spare key, and hide it someplace you're not likely to lose it - like your wallet?