like jayme said D11 is an input, so wont be able to control a relay etc like you want, but D12 is an output which is only used on HSV 6 speed manual, so you would be able to use it for your needs.
but I am guessing you will need some sort of table to setup when it comes on .... might be worth connecting that pin to a relay and a light, go for a drive and see if the light comes on on boost or similar.
No matter what the question is, the answer is always more horsepower!