Interesting thread. I am working on this too but with a 10.4" screen, not 12". It would be nice to use 12" because not many 10.4" screens are capable of 1024 res :(
The buttons are the trickiest part and finding concenrtic encoders for the knobs is proving hard work. In fact the button/knob issue wound me up so badly that I went and bought the original set from a Garmin distributor at great expense.
http://www.leons-world.com/img/g1000-cad_400.gif
http://www.leons-world.com/img/g1000_cad_600.gif
http://www.leons-world.com/img/g1000_but1_600.jpg
http://www.leons-world.com/img/g1000_but2_600.jpg
Chris