I'm not aware of another drain other than the petcock on the radiator. The correct GM way to fill the system is under a vacuum similar to the tool posted above by @lorennerol. As I understand it though, this is also not always a 100% method to eliminate air pockets. Due to the coolant air pocket issues, GM released a TSB to specifically address by installing 2 check valves.Not sure if thus question was asked.
Is there a coolant drain plug in the engine block that is accessible?
About air in the system....
Is there a low-point valve to drain the radiator like many vehicles have? If so, I wonder if you attach a hose there, raise the other end of the hose 6 feet high on a ladder with a funnel and add coolant from up there. This would allow coolant to fill from the bottom up to move the air up and out the top. There has to be an easy method that we are all missing here because when on the Kappa assembly line, they couldn't afford to fuss with air in the system.