The freezing in a block of ice method Shannen mentioned is what I use for saving animals that are going to be preserved for a scientific collection. For nectropsy, you don't want to freeze the animal.
Based on what you've said, I'm not sure he choked, at least not on the bark. Chameleons have their tracheal opening just behind the tongue. It looks like a small hole. For him to have choked and suffocated, something either would have had to be covering this opening of something small enough would have had to have been quickly inhaled into the lungs. Aspirating water is possible but aspiration of a foreign object seems unlikely due to the size of the trachea. I'll be interested to hear what the vet finds in the necropsy.
Chris