I've never heard of a rat making it that far down the digestive track and still have recognizable body parts. Snake poop looks like...poop. If it's NOT the spurs you're seeing, but is indeed some portion of rat body sticking out, I would immediately seek out the help of a vet accomplished in treating reptiles/snakes, because there'd be something far more going on than a mere case of constipation.

(Technical term is cloaca, by the way, in case you were curious! )