With BPs, I worry more about the safety of my snakes than my cat. Amos is definitely a hunter, and tends to kill anything he "plays" with. Even if he didn't, one good paw swipe could put out an eye, cause wounds that abcess, the list goes on. And if it's a big BP and a smaller cat, the tables could quickly turn the other way, where a vet visit becomes your best case scenario. And all because he was curious about the movey-thing...
As far as your question regarding the snake's behavior, it will be individually specific. I have some that wouldn't notice if my dog sat on them, and others that are tracking the cat the moment he enters the room.