It's shameful that GM was so terrified of liability that there's no mention of either release method in the Owner's Manual. Someone who's lost all power on the road is at the mercy of Bubba the Flatbed Driver. "Well, lady,your Hummer has a transmission lock and needs a special operation so's I can get it on the truck, and that'll cost you an extra hundred."
My Porsche's manual has detailed diagrams for releasing the trans and other vital release functions that an owner might need to use in a pinch.