The most laughable Youtube video about hail was shown recently. A guy is lying on top of his car in a violent hail storm 'swimming' about trying to sweep the hail stones off his car.
It's not the hail stones laying on his car that is going to do the damage but the new hail stones that will have a clear target to hit. In fact, a layer of hail stones will probably protect the car from additional hail because the new stones will impact the old ones, dissipating their energy.
A thick 'hail blanket' may not be a bad idea to protect your car...you just have to remember to bring your car's 'Blankee' with to work every day that it might storm.