The Vehicles & Measuring Distance section on pg 56 or the rulebook states "measure to and from the hull (ignore gun barrels, dozer blades, antennas, banners, and other decorative elements)." Why, oh why, couldn't they have added doors to that list.
The sections for vehicle sub-types are exceptions to the general vehicle rules. If the open top rules state any point on the vehicle, then they are referring to any point that is measured too on Pg. 56. So Battle-wagon deathrollas, drop pod doors, and boarding planks all don't count. They are not the "hull" of the model.
My position is that if someone insists that they count as a part of the model I'm going to refer back to Pg. 11: "A model may not move into or through the space occupied by another model (which is represented by its base or hull)...". So If I can't move across those doors, neither can the other player.