You could get hand operated wipers on jeeps for a long time.
I have not looked specifically at exemptions to the wiper law, but I know that seatbelt and lighting (turn signals, backup lights, etc) both have exemptions for vehicles that did not come from the factory with them. That is true in at least in the NHTSA regulations, CA, and ID because I have explictly checked them.
(Note: if you are in one of those states, verify that for yourself because the law could have changed since the last time I looked, especially in CA)
