I've done one of all of those options before. Most of my snakes nail the rat in about 5 seconds of putting it in the tub. Some of my snakes take their time and it takes a few hours or all night for them to decide that they want to eat. With those picker eaters, I ALWAYS place three rat food blocks in the corner of the tub of that snake for the rat. So in case the rat gets hungry during the hours or during the night, it has food for to nibble on.

Like said above, a well fed and watered rat, should not feel the need to attack or nibble on a snake. It more then likely will just make a "nest" away from the snake, and hang out there. Or at least that is what I have seen from all of the rats I've fed off or have seen be fed off at PKR.