The federal government is allowed to tell me I cannot carry a weapon onto a military base, and that seems to have past the constitutional muster. But, as a private property owner I cannot do the same thing?
I think the solution, for me, if I felt strongly about the issue, would be to tell all employees they cannot park on the property.
I am concerned about property rights, but , also, I don't understand why people feel the need to make this an issue. If you want to carry a gun in your car carry a gun. Don't advertise that fact. As long as it is not against state law it should never be an issue.