If this was a captive bred bunny...then I don't think there's much risk involved in feeding it off that way. Some BP's can be really picky eaters and if yours is a mouser, it may not even bother trying to eat anything else. But if it eats it easily...it's probably not a picky eater and will probably not have a problem the next time you feed it a mouse. Just be prepared for the small risk you run of having a few weeks of refusal, should the snake suddenly decide it really liked the baby rabbit above all else.