There is a guy on another forum that I belong to that appears to have hacked the GMLAN.
Post is Here
I am going to wait on the install of my car pc for a few weeks until this develops more. He started a week or two ago and has made lots of progress. If I am able to move the HVAC controls to a separate touch screen then I can replace my stock Head Unit with a Garrulity Screen in its place. This might help you out to ScrapMaker. We would not have to bother with the stock screen at all.