If a bicycle rack does not violate your state's, or any state that you will travel through, load projection laws then it should not matter what you place on the rack as long as it is properly secured and as Broctoon mentioned does not obscure tail/turn signals you should be fine. However the weight limit of a bike rack is based on using it as a bike rack and that is staggering the weight front to back. So a static weigh that is as far forward as possible is likely okay but a static weight at the end of the bars is probably not.