I now have a copy of 'The Battleship Yamato' by Janusz Skulski.
It is well worth the price, and shows fantastic details, far more than reviews mention.
Anyone wanting to know more about these great ships is well-advised to get a copy.
From page 100, figure E1/6, it looks like the only access to the gunhouses from the weatherdeck is partway up a ladder and through a single big armored door in the center of the back end of each gunhouse.
Was that the only normal way in or out from the weatherdeck, or was there any other access?