I've never had a problem positioning my holster so that it is accessible while wearing a seatbelt. This is true for both my Kholster and my Serpa, and in any vehicle. I may need to slide it forward a little bit so that the butt of the gun isn't pressing hard into the seat back. Beyond that, the buckle and belt typically ride right under and forward of the gun.
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.