If they do something for YOU then I am taking my Escalade in TOMORROW to have my either my differential or my A/C unit replaced. Your car is 3 years out of warranty and 8000 miles over. My 03 Escalade is only out of warranty by 6 months and I still have 46k!
That said, the Caddy dealers have a LITTLE more flexibility with regards to repairs now than they used to. However this is usually when youve bought the car or multiple cars FROM THAT DEALER and they know your gonna come back. They dont wanna waste their goodwill extension on just anyone.
I hope they grant you some courtesy extension, but I dont see how it would qualify when you just bought it used from a non-Cadillac dealer.
At this point Id recommend looking into that ebay sourced replacement motor. The truth is its the best way to go because they not only resolve the headgasket problem from EVER happening again, they ALSO take care of the OTHER known problems with that motor, OIL leaks at the half-case seal, and other common issues.
Good luck.