...As for cover garments being trapped by the seatbelt, I just make sure that doesn't happen. I usually pull my shirt or coat up and behind the grip so that the gun is visible before I buckle up. Even though the gun is exposed, it is very difficult to see, even for someone standing right outside the passengers window. If I do leave it covered, I still make sure that the cover garment and seatbelt aren't preventing access to the gun.
I do the same, whenever I'm carrying on my belt. I keep my coat unfastened and positioned so it isn't pinned to my body by the seatbelt, and all other clothing clear of the weapon. I sometimes drive through very high crime neighborhoods, due to the nature of my job, and occasionally steer with my left hand while my right rests on my gun butt.