If you really want to do that, better post a question on the
FIRST message board. Even if mounting the light different than upright was allowed in the past years, they probably changed the rule for a reason. Nevertheless, the seven updates so far show that they can still correct the rules.
By the way, answers to posts on the message board usually take awfully long, you probably won't have an answer until the robot is shipped. Think about mounteing the light upright and switching it at your first competition, provided that you are allowed to and have enough time.
Good luck!