We would not recommend using bulbs that are not legal for road use, even if the reversing lights are used rarely. It is worth pointing out that the function of a reversing light is to indicate to oncoming vehicles that a vehicle is reversing, not necessarily to illuminate the road behind you - if this were the case they would have the potential to dazzle oncoming traffic. We would advise fitting new bulbs of the correct original specification and ensuring the lights themselves are as clean as possible in order to ensure maximum visibility.