What idiot argued that an OSHA law would supersede a constitutional amendment when the interesting question is about a tension between pre-existing rights: the landowner's property rights versus the right to armed self-defense?
Thanks to the 14th Amendment, the question is now the same for a corporate parking lot versus my house: Do I have a right to preclude a person from bearing a weapon on my land or in my home?
The answer is a very simple, "Yes, you do." Keeping a weapon in the vehicle is not bearing in the sense that it is carried by or on a person. The minute someone steps out of the car carrying/bearing a weapon on YOUR parking lot, then you have a right to prosecute, fire, or discipline, as you please. The parking lot is yours; their car is theirs. No private property dispute at all.