i have been working on a prototype with a twist. instead of just pushbutton it is a fingerprint scanner. As i coded it now, it'll read your fingerprint and then enable a push button. Then you can crank the engine by pushing the button. After the engine is done cranking then you release the button. To turn it off, you press the button one more time.
This was before i was planning an ls swap and was worried about having cold starts and need to control how long you needed to crank. With the ls i plan on monitoring the tach signal and when the engine starts up it'll disengage the starter relay. If you're interested in something like this, i can share my project with you.
I'm open for ideas about how i should program the push button should work too.
here is a vid of my prototype:
https://www.youtube.com/watch?v=2lU41qzFbEY
another thing you can do instead of fingerprint is to use an rfid tag for your key.
they have a sticker rfid tag that you could possibly put on the backside of your alarm key fob
http://www.adafruit.com/products/362
or just get the keyfob rfid tag
http://www.adafruit.com/products/363
http://www.adafruit.com/products/1482