Snakes are escape artists. And they stress easily. The bag is both secure (if done right) and comforting to them. I would never, ever, transport a snake without it being secured in a bag...long trip or short.
Also, unless you're traveling in a vehicle that has no heat, there should be no reason to apply supplemental heat like you describe. Just keep the car comfortable for humans, and make sure the container the snake is in is well-insulated, and the snake should be fine. Hand warmers get WAY too hot and can be dangerous. Skip that huge risk and transport in a warm car.