A waiver offers protection against the normal hazards of parking, such as getting scratched by another car or by gravel kicked up by other vehicles, but would not protect against any unsafe conditions you knew or reasonable (that is logically) should have known about but failed to correct. So say that your field has a ditch beside it, which most parking lots don't, and the lip crumbled and a car slid into it--you could be sued despite a waiver. Or say it has ruts and holes in it, and someone turns an ankle--parking lots are usually smooth, so you could get sued. You would not be advised to do this without insurance.