You can open carry a pistol in Maine, but I have never seen anyone do it. I would apply for your CCW in Maine and then just carry concealed. When transporting firearms in Maine, you do have to have them unloaded. You cannot have them concealed on or about your person. They do not have to be locked or out of reach. The ammo does not have to be seperate.
So... in the unlocked glove compartment is illegal. On the seat next to you covered with a coat is illegal.
Sitting on the seat next you unloaded with a loaded mag next to it is legal.
Sitting on the seat next to you in a case unlocked would be illegal.
When I travel, they are unloaded and 1) in an unlocked case in the trunk or backseat or 2) not in a case and sitting on the back seat or right next to me on the front passenger seat.
Again, the key things are unloaded and not concealed on or about your person.
This question gets asked a lot by MA folks. If you scroll down this forum you will find a similar posting with the laws cited and linked. That way you can read for your self what the law says and not trust what I am saying. Have fun in Maine, it is a great place for MA people to visit.