Here's my take on doing that. Say you work for a private nongovernmental employer (this exempts me as a school teacher) and your employer places a private ban on guns in parked cars. Folks who are holders of concealed wqeapons permits can sue if they are injured by a criminal attack on their way to or from work. It's a case with "legs" because it would bring up both of the main justifying processes of tort law: The "foreseeability test" That a criminal attack was foreseeable and preventable and the second part which is the crucial but for the actions of the defendant said victim of such attack herein known as the plaintiff, could have had at least a chance of avoiding victimization. I think that such a lawsuit would even trump a pre-agreed to document stating conditions of employment. In similar fashion that a pre-nuptial agreement can be broken.
I know just about nothing about tort law. But yes, I can see where one could sue their employer by demonstrating that he was a crime victim due to a company policy of banning firearms from cars in the lot. OTOH, if someone went postal, couldn't the reverse form the basis of a suit?