I could keep a Hustler magazine in the car if it wasn't lying on the seat open to a "pictorial" page. Not sure what would happen if it were and I don't want to find out.
The point I should have made is not that my employer restricts my rights, it's that I surrender some of my rights by working there. Look at the military contract. It's even more restrictive. They can't publicly criticize the chain of command. They can't refuse a lawful assignment. Why they can't even commit adultery! - CINC excepted of course if (D)
My point is similar to yours.
Employers can not regulate what publications you have in your car, and to be consistent, can not regulate if you have a firearm in your car. And... I agree that you can not have anything like a gun or a hustler displayed openly.
That said, I do not advocate having a firearm in your car on a military installation, in Massachusetts or any other state that has restrictive 2nd amendment laws.